Intensifying its campaign to terrorise the U.S. with its bellicose threats, North Korea on Saturday released a dramatic propaganda video called "Last Chance" depicting a nuclear strike on Washington D.C., complete with animation of a ballistic missile slamming near the Lincoln Memorial.

The four-minute video shows a mushroom cloud following the nuclear strike and an American flag going up in flames, USA Today reported.

"If the American imperialists provoke us a bit, we will not hesitate to slap them with a pre-emptive nuclear strike," read the Korean subtitles in the video, according to The New York Times. "The United States must choose! It's up to you whether the nation called the United States exists on this planet or not."

The video was uploaded on YouTube even as a renowned expert on the Far East warned the U.S. not to underestimate North Korean dictator Kim Jong-un.

Columnist and author Gordon Chang—who has studied China and North Korea up close for years and is author of "Nuclear Showdown: North Korea Takes on the World"—said the U.S. should pay attention to the North Korean sabre-rattling, WND reported.

The North Korean missile tests and the detention of an American college student ought to be taken very seriously by U.S. President Barack Obama and other world leaders, Chang said.

In recent days, North Korea test-fired numerous ballistic missiles in an obvious attempt to intimidate its traditional foes — South Korea, Japan and the U.S. North Korean authorities also sentenced University of Virginia student Otto Warmbier to 15 years of hard labour for tearing down a poster of the late Kim Jong-Il inside a North Korean hotel.

The missile tests followed a statement from Kim that he wanted the military to have nuclear weapons ready for use "at any time."

"What we're concerned about, however, is their testing of long-range missiles," Chang said. "They do have two launchers that can reach 48 states. Therefore, we've got to be concerned because, eventually, if not now, they'll be able to put nuclear warheads on those missiles and, therefore, threaten America."

On Thursday, reports surfaced that U.S. intelligence analysts believe North Korea "probably" already has a miniaturised nuclear warhead.

North Korea has repeatedly issued nuclear strike threats against both Seoul and Washington.

On Saturday, North Korea warned that it is ready to strike the South Korean presidential palace unless South Korea's president apologises for "treason" and "publicly executes" officials responsible for what Pyongyang says are plans to attack its leadership, the North's official Korean Central News Agency reported.