TOKYO (Reuters) - Japan has upgraded its estimate of North Korea’s nuclear weapons capability in an upcoming annual Defence White Paper, saying it seems Pyongyang has achieved the miniaturization of warheads, the Yomiuri newspaper said in an unsourced report on Wednesday.

FILE PHOTO: North Korea's leader Kim Jong Un supervises a "strike drill" for multiple launchers and tactical guided weapon into the East Sea during a military drill in North Korea, in this May 4, 2019 photo supplied by the Korean Central News Agency (KCNA). KCNA via REUTERS

That compares with the assessment in last year’s report in which the government said it was possible North Korea had achieved miniaturization, 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.

North Korea’s ability to build nuclear warheads small enough to fit on its ballistic missiles has been widely accepted for several years, but the Japanese report highlights the lack of progress on denuclearisation talks aimed at curtailing the program, said Vipin Narang, a nuclear affairs expert at the Massachusetts Institute of Technology in the United States.

“It is Japan that is most threatened, and probably the primary target of such a capability,” he said. “So openly acknowledging it underscores Tokyo’s acute fears that North Korea’s nuclear program continues to grow unabated with no foreseeable plan to slow its growth, let alone eliminate them.”

South Korea’s 2018 Defence 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.

U.S. envoy on North Korea Stephen Biegun was in Seoul this week to meet with South Korean officials, and said he was prepared to engage with North Korea “as soon as we hear from our counterparts.”

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 “miniaturizing 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 miniaturization of nuclear weapons and has developed nuclear warheads.”