Japan has upgraded its estimate of North Korea's nuclear weapons capability in an upcoming annual Defence White Paper, saying it seems Pyongyang has already achieved the miniaturisation of warheads, the Yomiuri newspaper said in an unsourced report on Wednesday.

That compares with the assessment in last year's report in which the government said it was possible North Korea had achieved miniaturisation, the Japanese daily said without citing sources.

The report, to be approved at a Cabinet meeting in mid-September, will maintain the assessment that North Korea's military activities pose a "serious and imminent threat", the Yomiuri said.

South Korea's 2018 Defense White Paper, released in January, reported that North Korea’s ability to miniaturize nuclear weapons "appears to have reached a considerable level."

According to South Korean media reports late last year, the South Korean intelligence agency told lawmakers that North Korea had continued to miniaturize nuclear warheads even after the Singapore summit between Trump and Kim in June 2018.

At that time, North Korea committed "to work toward complete denuclearisation of the Korean peninsula" and destroyed some tunnels and buildings at its Punggye-ri nuclear test site.

But a second Trump-Kim meeting in February collapsed without an agreement, and North Korea has since resumed missile tests.

American officials have concluded for years that North Korea had likely produced miniaturized nuclear warheads. A leaked report by the Defense Intelligence Agency in 2017 concluded that North Korea had successfully produced a miniaturized nuclear warhead that can fit inside its missiles, according to The Washington Post.

In last year's Defence White Paper, Japan said "miniaturising a nuclear weapon small enough to be mounted on a ballistic missile requires a considerably high degree of technological capacity," and that "it is possible that North Korea has achieved the miniaturisation of nuclear weapons and has developed nuclear warheads."