President Donald Trump called off Secretary of State Mike Pompeo's trip to North Korea after receiving a mysterious and hostile letter from Kim Jong Un's government, The Washington Post reported.

Officials declined to comment to The Post on the contents of the message and how it was transmitted.

On Sunday, North Korea's state newspaper accused Washington of plotting to "unleash a war" on Pyongyang while continuing to negotiate "with a smile on its face."

President Donald Trump reportedly called off Secretary of State Mike Pompeo's trip to North Korea after receiving a mysterious and hostile letter from Kim Jong Un's government.

Pompeo received the letter on Friday morning from Kim Yong Chol, the vice chairman of North Korea's ruling Workers' Party Central Committee, then showed it to Trump, The Washington Post reported on Monday, citing two senior Trump administration officials.

Officials declined to comment to The Post on how the letter was transmitted. And while the contents of the message are unclear, it was apparently hostile enough for Trump to nix Pompeo's travels and issue a series of tweets.

"Secretary Pompeo looks forward to going to North Korea in the near future, most likely after our Trading relationship with China is resolved," Trump tweeted. "In the meantime I would like to send my warmest regards and respect to Chairman Kim. I look forward to seeing him soon!"

The White House press secretary, Sarah Huckabee Sanders, told CNN on Saturday that Pompeo was in the room when Trump published the tweets.

On Sunday, North Korea's state newspaper, Rodong Sinmun, responded to Pompeo's abrupt cancelation and accused Washington of plotting to "unleash a war" on Pyongyang while continuing to negotiate "with a smile on its face."

According to KCNA Watch, the newspaper described "extremely provocative and dangerous" US military movements in the region and claimed that US "special units" in Japan staged a flying exercise in the Philippines simulating "the infiltration into Pyongyang."

It also claimed that the USS Michigan, a nuclear US Navy submarine, transported Green Berets and other special forces from Okinawa, Japan, to the Jinhae Naval Base in South Korea in late July or early August.

Relations between North Korea and the US recently hit a roadblock when the US asked North Korea to reveal what the US says are secret nuclear facilities, which North Korea has denied having. Meanwhile, satellite imagery of North Korea has suggested that it might be continuing or increasing its production of nuclear material.