President Recep Tayyip Erdogan on Wednesday announced the beginning of Turkey's operation in northeastern Syria, three days after President Donald Trump agreed to move U.S. troops out of the region to clear the way for Ankara.

"The Turkish Armed Forces, together with the Syrian National Army, just launched Operation Peace Spring," Erdogan tweeted. "Our mission is to prevent the creation of a terror corridor across our southern border, and to bring peace to the area."

Meanwhile, the Kurdish-led Syrian Democratic Forces — a key U.S. ally in the war on the Islamic State group that controls much of the area close to the border — said airstrikes had begun.

"There is a huge panic among people in the region," spokesman Mustafa Bali tweeted.

NBC News could not independently confirm airstrikes.

The Syrian Democratic Forces, or SDF, are led by the Kurdish YPG militia, which has long angered the Turkish government and Erdogan. Turkey sees the YPG as an extension of the separatist Kurdistan Workers’ Party, which is considered a terrorist group by the U.S.

In a statement earlier on Wednesday, the SDF called on the international community and all countries in the international coalition against ISIS to “carry out their responsibilities and avoid a possible impending humanitarian disaster."

"This attack will spill the blood of thousands of innocent civilians because our border areas are overcrowded," it said.

According to multiple current and former U.S. officials, the White House's announcement blindsided not just America's Kurdish partners but almost everyone — senior officials at the Pentagon, the State Department and the White House, lawmakers on Capitol Hill, and U.S. allies in Europe and the Middle East.

On Monday, Trump warned in a tweet that if Turkey — a longtime American ally with NATO's second-largest standing army — did anything that he considered to be "off limits," he would "totally destroy and obliterate" its economy.

The White House said in a statement on Sunday that Turkey would now be responsible for all ISIS fighters in the area captured over the past two years.