The United States announced it will not be suspending any more "war games" on the Korean peninsula amid souring relations with Pyongyang.
In the wake of Donald Trump’s summit with Kim Jong-un on June 12 the US had suspended some of this year’s major military exercises, planned in conjunction with South Korea, as an act of "good faith".
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But Jim Mattis, the US defence secretary, said on Tuesday night: "We took the step to suspend several of the largest military exercises as a good faith measure. At this time there is no discussion of further suspensions."
He gave no indication when US military exercises may resume, adding: "We are going to see how the negotiations go, and then we will calculate the future, how we go forward."
His comments came as North Korea warned in a letter to the US that denuclearisation talks were "again at stake and may fall apart".
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The letter suggested that, if progress was not made, North Korea could resume "nuclear and missile activities".
It was delivered directly to Mike Pompeo, Mr Trump’s secretary of state, and criticised the US for not agreeing to a peace treaty officially ending the Korean War.
"The US is still not ready to meet (North Korean) expectations in terms of taking a step forward to sign a peace treaty," the letter said.
The Korean War ended in an armistice, and North Korea has argued that an end to the technical state of war is key to lowering tensions on the peninsula.
But the US has been reluctant to go forward with a peace treaty before North Korea denuclearises.
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After receiving the North Korean letter Mr Trump cancelled a trip Mr Pompeo was due to make to Pyongyang.
Meanwhile, Japan said it still believed North Korea posed an "an unprecedentedly serious and imminent threat".
Its first annual defence review since the recent easing of tensions Tokyo said: "There is no change in our basic recognition concerning the threat of North Korea’s nuclear weapons and missiles."
For years US and South Korean forces have been training together including for beach landings, an invasion of North Korea, and "decapitation" strikes against the Kim regime.
Exercises suspended by the US in the wake of the Singapore summit included the large-scale Ulchi Freedom Guardian, which had been due to take place this month with 17,500 US military personnel.
Mr Mattis declined to say whether he thought resuming future exercises could be provocative.
He said: "Even answering a question in that manner could influence the negotiations. Let’s let the negotiations, let the diplomats go forward."
He said there had been "progress" in Singapore but denuclearisation of North Korea was always "going to be a long and challenging effort".