By Courtney Kube and Ian Johnston, NBC News
(NBC) -- North Korea put its rocket units on standby Friday to attack U.S. military bases in South Korea and the Pacific, after repeated threats and one day after two American stealth bombers flew over the Korean Peninsula in a military exercise.
A U.S. official warned that the isolated communist state is "not a paper tiger" and its reaction should not be dismissed as "pure bluster."
According to the North Korea's official KCNA news agency, the country's leader Kim Jong Un "judged the time has come to settle accounts with the U.S. imperialists in view of the prevailing situation" at a midnight meeting of top generals, Reuters reported.
The latest threat comes one day after two nuclear-capable stealth bombers flew from Missouri to drop inert munitions on a range in South Korea as part of a major military exercise.
The U.S. official emphasized the danger posed by North Korea's military and the unpredictable nature of its 30-year-old leader.
"North Korea is not a paper tiger so it wouldn't be smart to dismiss its provocative behavior as pure bluster. What's not clear right now is how much risk Kim Jong Un is willing to run to show the world and domestic elites that he's a tough guy," said the official, who asked not to be named. "His inexperience is certain -- his wisdom is still very much in question."
There was a mass demonstration in support of Kim involving tens of thousands of people in the main square of North Korean capital Pyongyang Friday, The Associated Press reported.
Placards read "Let's crush the puppet traitor group" and "Let's rip the puppet traitors to death!"
The state-controlled KCNA also published an article that said the "opportunity for peacefully settling the DPRK-U.S. relations is no longer available as the U.S. opted for staking its fate. Consequently, there remains only the settlement of accounts by a physical means." DPRK stands for Democratic People's Republic of Korea, the North's official name.
"A battle to be fought by the DPRK against the U.S. will become a war for national liberation to defend the sovereignty and dignity of the country and, at the same time, a revolutionary war to defend the human cause of independence and the justice of the international community," the article by "news analyst" Minju Joson said.
South Korea's Yonhap news agency quoted a South Korean military official as saying that there had been "increased movement of vehicles and forces" at missile launch sites in the North. "We are closely watching possibilities of missile launches," the unnamed official said.
North Korea routinely issues hostile statements but analysts have noted recent remarks have become more belligerent. In December, the North carried out a long-range rocket test and then detonated a nuclear bomb in a test earlier this year.
At a daily news briefing Friday, China's Foreign Ministry Spokesman Hong Lei said China was calling for an easing of tensions.
But some fear the situation could be getting out of control.
"It seems that Kim Jong Un is in the driving seat of a train that has been taken on a joyride," Lee Min-yong, an expert on North Korea at Sookmyung Women's University in Seoul, told Reuters.
Russia, meanwhile, appeared to criticize the U.S. over Thursday's bomber mission.
"We are concerned that alongside the adequate, collective reaction of the U.N. Security Council, unilateral action is being taken around North Korea that is increasing military activity," Foreign Minister Sergei Lavrov told reporters in Moscow, according to Reuters.
"The situation could simply get out of control; it is slipping toward the spiral of a vicious cycle," he said.