US President Donald Trump has issued his strongest warning ever in response to North Korea's escalating nuclear threat.

Trump is reported to have said "they will be met with fire and fury like the world has never seen".

The warning followed news that North Korea may have successfully produced a miniaturised nuclear warhead that can fit inside its missiles, passing a key threshold in becoming a full-fledged nuclear power, according to a Japanese defence paper and a US media report.

The UN Security Council this weekend slapped its toughest sanctions yet on North Korea over its latest test of a ballistic missile that could be used to deliver a nuclear weapon.

Despite the rapid tempo of these tests, uncertainty has lingered over the isolated nation's ability to couple such a missile with a nuclear device.

Those uncertainties appear to be receding.

Japan's Defence Ministry concluded in an annual white paper released Tuesday that "it is possible that North Korea has achieved the miniaturisation of nuclear weapons and has developed nuclear warheads."

Japan, a key US ally, is also a potential target of North Korean aggression.

And The Washington Post reported Tuesday that US intelligence officials assess that a decade after North Korea's first nuclear test explosion, Pyongyang has produced nuclear weapons for ballistic missile delivery, including by intercontinental missiles - the type capable of reaching the continental US.

The Post story, citing unnamed US intelligence officials, said the confidential analysis was completed last month by the US Defence Intelligence Agency. The US also calculated last month that North Korea has up to 60 nuclear weapons, the Post said, more than double most assessments by independent experts.

Officials at the agency would not comment Tuesday on the report. The Office of the Director of National Intelligence could not immediately be reached for comment.

larm in Washington over North Korean leader Kim Jong Un's pursuit of a nuclear capability has intensified in the past month after the North conducted two tests of intercontinental ballistic missiles for the first time last month.