UPDATE: Iran vowed to seek revenge on the United States on Friday after an American airstrike killed one of the country's most powerful military and political figures.

The U.S. bombing near Baghdad airport killed Qassem Soleimani, the high-profile commander of Iran's secretive Quds Force and sometimes described as the country's second most influential person.

"The great nation of Iran will take revenge for this heinous crime," Iranian President Hassan Rouhani tweeted after the news was confirmed.

Iran's Supreme Leader Ayatollah Ali Khamenei said a "harsh retaliation is waiting for the criminals whose filthy hands spilled his blood." The country's foreign minister, Javad Zarif, called the U.S. strike "an act of state terrorism and a violation of Iraq's sovereignty."

Soleimani was one of the most influential figures in the Middle East, having developed a network of powerful militia groups whose clandestine reach stretched into Iraq, Syria and beyond.

His death is a major escalation in the recent tensions between Washington and Tehran, and Friday morning local time, the State Department urged all U.S. citizens "to depart Iraq immediately" because of the increased risk.

The strike was directed by President Donald Trump; House Speaker Nancy Pelosi, D-Calif., called on the administration to immediately brief lawmakers. She said the bombing was carried out without "authorization for use of military force" against Iran and without the consultation of Congress.

Multiple domestic U.S. law enforcement agencies said they were keeping a watchful eye on events overseas and ready to react as necessary.

The strike was met with caution by the United Kingdom, one of Washington's biggest allies that in the past has backed it in the Middle East and elsewhere.

"We have always recognized the aggressive threat posed by the Iranian Quds Force led by Qassem Soleimani," British Foreign Secretary Dominic Raab said. "Following his death, we urge all parties to de-escalate. Further conflict is in none of our interests."

France's deputy minister for foreign affairs, Amelie de Montchalin, told her country's RTL radio station that the airstrike meant "we are waking up in a more dangerous world."

Secretary of State Mike Pompeo said in a series of tweets that he has been calling world leaders to reassure them that the "U.S. remains committed to de-escalation." But asked on CNN about the reaction from Paris, Pompeo said, "The French are just wrong about that."

Israeli Prime Minister Benjamin Netanyahu is cutting short his visit to Greece and was due back in Israel Friday.

"Just as Israel has the right of self-defense, the United States has exactly the same right," he said, adding that Trump "deserves all the credit for acting swiftly, forcefully and decisively."

In addition to being Iran's most senior general, Soleimani also held huge political clout, with many considering his influence second only to that of the supreme leader himself.

His Quds Force is a unit in Iran's powerful Revolutionary Guard and classified as a terrorist group by the U.S. and others.

The Iranian militias he cultivated are "responsible for the deaths of hundreds of American and coalition service members and the wounding of thousands more," according to the Department of Defense, which confirmed his death early Friday local time.

Hours after Soleimani's death, Iran announced that Brig. Gen. Esmail Ghaani would replace him as head of the Quds Force, according to the country's semi-official ISNA news agency.

Another man, Abu Mahdi al-Muhandis, said to be the deputy of the militias' umbrella group and a close adviser to Soleimani, was also killed in the U.S. airstrike, according to Iraqi television reports.

The deaths drew strong censure from Iraq's caretaker prime minister, Adel Abdul-Mahdi, who expressed his "utmost condemnation of the American administration’s assassination of the two martyrs" and called them "great symbols in achieving victory against the terrorist group ISIS."

The U.S. has around 5,000 troops in Iraq at the invitation of the government, their brief to assist government troops in the fight against the Islamic State militant group.

But the Iraqi government also recognizes and supports the militia groups, many of which are backed by Iran.

The country's prime minister called Soleimani's killing "a flagrant violation of the conditions for the presence of the American forces in Iraq,'' pointing out that "its role is limited to training Iraqi forces and fighting ISIS within the international coalition forces, under the supervision and approval of the Iraqi government."

Iraqi President Barham Salih told NBC News that "the situation in Iraq is very fragile, very precarious."

He said, "I urge restraint on all sides. Iraq cannot be condemned to another cycle of violence. We have had too many wars over the last four decades. The situation now is very dangerous, no doubt, and that is why I am urging calm."

The airstrikes also prompted some celebrations in Baghdad on Friday, with Pompeo tweeting a video that he said showed Iraqis "dancing in the street for freedom."

In recent months, protests have erupted in the Iraqi capital. The demonstrators are angry at corruption and unemployment but also what they see as the complicity of neighboring Iran in their affairs.

Soleimani was a key figure in his country's involvement in Syria, where it was allied with Russia in supporting President Bashar al-Assad.

Russia's foreign ministry called the killing "an adventurist step" by the U.S. that would lead to increased tensions throughout the region, according to the Interfax news agency.

Meanwhile China urged "all parties concerned, especially the United States, to maintain calm and restraint and to avoid the further escalation of tension," foreign ministry spokesman Geng Shuang told a press briefing Friday.

PREVIOUS STORY: Iraq President Barham Saleh told NBC News Chief Foreign Correspondent Richard Engel “The situation in Iraq is very fragile, very precarious. I urge restraint on all sides. Iraq cannot be condemned to another cycle of violence. We have had too many wars over the last four decades. The situation now is very dangerous no doubt and that is why I am urging calm.”

PREVIOUS STORY: The United States killed a high-profile commander of Iran’s secretive Quds Force, the Department of Defense said late Thursday.

"At the direction of the President, the U.S. military has taken decisive defensive action to protect U.S. personnel abroad," the department said in a statement.

Another man said to be the deputy of militias known as Popular Mobilization Forces was also killed in the strike that killed Qassem Soleimani — an iconic military commander of Iran's military forces in Iraq, Syria, Lebanon and throughout the Middle East, according to Iraqi Television reports.

In the past, the U.S. has credited Soleimani 's militias with combating an American enemy — ISIS in Iraq. Soleimani's Quds Force was a division of Iran's Revolutionary Guard, widely believed to support a wide number of Iranian-backed terror groups such as Hezbollah.

"This strike was aimed at deterring future Iranian attack plans," the Defense Department said in its statement. "The United States will continue to take all necessary action to protect our people and our interests wherever they are around the world."

The Pentagon said that Soleimani was actively developing plans to attack American diplomats and service members in Iraq and throughout the region.

His death is likely to dramatically increase tensions between the U.S. and Iran, which were already heightened by the New Year's attacks on the American Embassy in Baghdad.

Rioters for two days attempted to scale the fortress-like walls at the world's largest embassy. They retreated under a show of force from the Pentagon; 100 Marines were airlifted into the compound and about 700 more Army paratroopers are expected soon in Kuwait from a global response force based at Fort Bragg in North Carolina.

Earlier, Defense Secretary Mark Esper threatened a pre-emptive strike against Iranian militia if there were any renewed attacks against U.S. personnel or interests in Iraq.

The Chairman of the Joint Chief told reporters attackers would run into a buzz saw.

Al-Muhandis was one of the closest Iraqis to Iran’s Islamic Revolutionary Guard Corps, and he had been sanctioned by the U.S. Treasury for acts of violence against Americans. He was also considered a close adviser to Soleimani.

While U.S. strike comes amid heightened tensions between the U.S. and Iran over rocket attacks in Iraq that U.S. officials had blamed on Iranian-backed forces and the attempted breach of the U.S. Embassy in Baghdad.

The conflict at the embassy occurred after U.S. fighter jets on struck weapons depots in Iraq and Syria that the United States said were linked with a group called Kataeb Hezbollah, which it blames for attacks on bases of the U.S.-led coalition fighting the Islamic State in recent months.

At least 25 militia fighters were killed in the airstrikes. The strikes followed the death of a U.S. contractor who was killed in a rocket attack on an Iraqi military base in Kirkuk that also hosted coalition forces Dec. 27. Several U.S. members were also injured.

The Defense Department said Thursday in announcing the strike that killed Soleimani that he had orchestrated attacks on coalition bases in Iraq over several months, including the Dec. 27 attack that killed the contractor.

He “also approved the attacks on the U.S. Embassy in Baghdad that took place this week,” the Defense Department said in the statement.

Trump has not appeared to comment on the strike but tweeted an image of an American flag with no text.

Earlier this week, when asked about the possibility of war with Iran, Trump said that "I don’t see that happening." "No, I don’t think Iran would want that to happen."

"I want to have peace; I like peace. And Iran should want peace more than anybody," Trump told reporters at his New Year’s Eve event at Mar-a-Lago in Florida.

At least one U.S. Congressman expressed alarm at the reports.

Sen. Chris Murphy, D-Connecticut, who sits on the Senate Committee on Foreign Relations, tweeted: "Soleimani was an enemy of the United States. That’s not a question."

"The question is this — as reports suggest, did America just assassinate, without any congressional authorization, the second most powerful person in Iran, knowingly setting off a potential massive regional war? Murphy said in the tweet.

Sen. Marco Rubi, R-Florida, who also sits on the Foreign Relations Committee, tweeted that the U.S. and Trump "exercised admirable restraint while setting clear red lines and & the consequences for crossing them" following what he said were repeated attacks by the Islamic Revolutionary Guard Corps.

"#Iran's Quds Force chose the path of escalation," Rubio said in the tweet. "They are entirely to blame for bringing about the dangerous moment now before us."

The Trump administration in April announced it was designating the Islamic Revolutionary Guard Corps as a foreign terrorist organization, which is the first time the U.S. had used that designation on part of another country’s government.

Since then there have been several tense incidents involving the U.S. and Iran, including when Iran in June shot down a U.S. surveillance drone that Iran’s Islamic Revolutionary Guard Corps said entered Iranian airspace. U.S. Central Command said the aircraft was in international airspace.

The Trump administration also blamed Iran for a September attack on oil sites in Saudi Arabia, which prompted the U.S. to deploy military forces to the Middle East that Esper said at the time would be defensive in nature.