This summer, the Japanese concealed a meeting they held with North Korea from senior USA officials, according to people familiar with the matter.
North Korea analyst Robert Kelly of Pusan University said Trump's tweets were a clown show.
"It would be an overstatement to say that the U.S. is trying to break apart the South-North relations, but it is a message that it is not comfortable with the current situation, knowing that the North has been reportedly violating sanctions", the Seoul-based expert added.
The White House, in a statement sent on Twitter by Mr Trump, said the US President believes North Korea is under "tremendous pressure" from China but that Beijing also was supplying Pyongyang with "considerable aid", including fuel, fertiliser and commodities. Senior U.S. officials expressed irritation that Japan wasn't forthright about the meeting, given Washington's near-constant updates to Tokyo on its dealings with North Korea.
However, he signaled that he could restart them if he wanted to, promising that they would "be far bigger than ever before".
U.S. Defense Secretary Jim Mattis on Wednesday clarified his previous statements on the fate of additional large-scale exercises on the Korean Peninsula, leaving open the possibility of future cancellations. Mattis said the "Foal Eagle" military exercise planned for next March hasn't been called off, . and may resume pending negotiations with North Korea.
President Donald Trump in June suspended what he called "war games" with Seoul, saying he believed Kim "wants to get it done" on denuclearization.
The move was broadly criticized as a premature concession to North Korea, which has resisted USA efforts to persuade it to give up its nuclear weapons.
Pompeo was scheduled to visit North Korea again this week before the trip was cancelled last minute.
Trump said earlier on Wednesday negotiations with North Korea over the nation's nuclear weapons programme are "doing well" but an ongoing trade dispute with China is getting in the way. Sure, in some instances it's a good thing for the US colonialist state to lose some of its power over smaller nations, but stuff like this is incredibly risky.
The war games may be back on. -China trade disputes, and other differences, they will be resolved in time by President Trump and China's great President Xi Jinping'.
The United Nations Command declined the two Koreas' request to test-run a train on a cross-border railway last week, sources said Thursday, fueling speculation that the USA might be reluctant to support inter-Korean projects amid stalled denuclearization talks with North Korea. It has also suspended nuclear tests and test-missile firings.
A week later, Mattis said the exercises had been paused to "create space for our diplomats to negotiate strongly, and increasing the prospects for a peaceful solution on the peninsula".
For North Korea, China is its only major ally and the main transit for any goods entering Pyongyang, according to AFP.
"I don't have a crystal ball right now", he said. He said that the tone of the letter was "if you're not willing to give something, then don't come".
"I don't know if it is off the rails as much as they aren't making progress on key issues".