North Korea put its rocket units on standby Friday to attack U.S. military bases in South Korea and the Pacific, after repeated threats 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 South Korea's news agency, Yonhap, North Korea announced Saturday that it had entered a state of war against South Korea. "In a special statement, the North said it will deal with every inter-Korean issue in a wartime manner," Yonhap reported. NBC News could not immediately confirm.

The two Koreas have been in a technical state of war because their 1950-53 conflict ended under an armistice and not a peace treaty.

NBC's Andrea Mitchell examines North Korea's brewing threats and what they mean for neighboring South Korea.

According to 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 threats come 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!"

'War for national liberation'
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.