President Donald Trump canceled Secretary of State Mike Pompeo's trip to North Korea after the latter sent a "belligerent" letter to the administration warning that talks are in jeopardy and that the North may resume "nuclear and missile activities."

The US is frustrated by North Korea's failure to move forward on denuclearization of the Korean Peninsula, and North Korea is irritated by Washington's unwillingness to sign a peace treaty ending the Korean War.

Secretary of Defense Jim Mattis left the door open Tuesday for the possible resumption of joint war games with South Korea, stressing that there are currently no new plans to suspend additional drills beyond the large exercises canceled this year.

North Korea warned the US in a recent letter that talks are "again at stake and may fall apart," adding that it may resume "nuclear and missile activities" if its demands are not met.

President Donald Trump unexpectedly canceled what was expected to be Secretary of State Mike Pompeo's fourth trip to Pyongyang last Friday due to insufficient progress on the denuclearization of the Korean Peninsula. The decision was preceded by a "belligerent" letter that criticized his administration for failing "to meet (North Korean) expectations in terms of taking a step forward to sign a peace treaty," CNN reported Tuesday, citing people familiar with the matter.

The receipt of the letter, which was sent by the former head of North Korea's spy agency, Kim Yong Chol, occurred just hours after Pompeo's trip was first announced last week, The Washington Post reported Monday. "The exact contents of the message are unclear, but it was sufficiently belligerent that Trump and Pompeo decided to call off Pompeo’s journey," The Post's Josh Rogin reported.

Pompeo's last trip to North Korea ended with a message from the foreign ministry characterizing meetings with the US as "regrettable." Those negotiations came amid troubling reports from multiple outlets indicating that North Korea had yet to suspend its weapons programs in keeping with its commitment to denuclearize.

In recent months, media reports have indicated that North Korea is making infrastructure improvements at nuclear reactors, research facilities, and missile development sites and increasing the production of fuel for nuclear weapons. The North has also reportedly halted the dismantlement of a key facility Kim promised to destroy as a concession to Trump in Singapore.

Over the past few weeks, North Korean media has railed against US attitudes and actions, especially the sanctions that continue to hobble North Korea's limited economy.

Speaking to the press at the Pentagon Tuesday, Defense Secretary Jim Mattis refused to suggest that North Korea is acting in bad faith, but he left the door open to the possibility of restarting war games should North Korea's behavior warrant such a step.

"As you know, we took the step to suspend several of the largest exercises as a good faith measure. We have no plans at this time to suspend any more exercises," he said at the briefing. Emphasizing that his team will work closely with the secretary of state, he explained that "at this time, there has been no discussion of further suspensions."

Mattis added that there are smaller exercises ongoing on the peninsula at all times. "The reason you've not heard much about them is [so] North Korea could not in any way misinterpret those as somehow breaking faith with the negotiation," he told the media.

Pentagon officials told Business Insider that there are numerous exercises happening all the time as South Koreans and US personnel train together to enhance their interoperability.

During the briefing, the secretary and Chairman of the Joint Chiefs of Staff Gen. Joseph Dunford said they would let diplomacy lead, stressing that they did not want their comments to influence negotiations. "We stay in a supporting role," Mattis noted.

Mattis said this would be a "long and challenging effort."

The recent moves and comments from both sides indicate that there is growing frustration between Pyongyang and Washington. For the time being, it appears that North Korea is resistant to denuclearization and the US is hesitant to sign a peace treaty ending the Korean War without those disarmament steps.