Updated: President Donald Trump has said North Korea "had best not make any more threats to the United States" or "they will be met with fire and fury like the world has never seen".
Mr Trump issued the warning during a briefing on opioid addiction at his golf course in Bedminster, New Jersey.
A Japanese defence paper and a US media report said that North Korea may have successfully produced a miniaturised nuclear warhead that can fit inside its missiles.
Japan, a key US ally, is also a potential target of North Korean aggression.
A report in The Washington Post on Tuesday went further.
The newspaper said US intelligence officials have assessed 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.
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, but those uncertainties appear to be receding.