The United States has announced it would not suspend any more "war games" on the Korean Peninsula amid souring relations with Pyongyang.

America had suspended some of this year's military exercises with South Korea in the wake of US President Donald Trump's summit with Kim Jong Un, the North Korean leader, in June.

However, Jim Mattis, the US Defence Secretary, said: "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 did not say when military exercises would resume, adding: "We are going to see how the negotiations go, and then we will calculate the future."

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His comments came as North Korea warned in a letter to the US that denuclearisation talks were "again at stake and may fall apart" and that if progress was not made, North Korea could resume "nuclear and missile activities".

It was delivered directly to Mike Pompeo, Trump's Secretary of State, and criticised the US for not agreeing to a peace treaty officially concluding the Korean War, which ended in an armistice.

North Korea argued an end to the technical state of war was key to lowering tensions on the peninsula. But the US had been reluctant to do this before North Korea denuclearised.

After receiving the North Korean letter, Trump cancelled a trip Pompeo was due to make to Pyongyang.

Meanwhile, Japan said it still believed North Korea posed an "an unprecedentedly serious and imminent threat".

In its first defence review since the June summit, Tokyo said: "There is no change in our basic recognition concerning the threat of North Korea's nuclear weapons and missiles."

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 involving 17,500 US military personnel.

Mattis declined to say whether he thought resuming future exercises could be provocative. He added there had been "progress" in Singapore but denuclearisation of North Korea was always "going to be a long and challenging effort".