President Donald Trump has warned North Korea that it could face "fire and fury" after a new report said US intelligence believes Pyongyang has successfully produced a nuclear warhead that can fit inside its missiles.
Washington's alarm over North Korean leader Kim Jong Un's pursuit of a nuclear capability has intensified after the North conducted two tests of intercontinental ballistic missiles last month.
The latest report that it has produced a miniaturised nuclear warhead would mean North Korea has passed a key threshold in becoming a fully fledged nuclear power.
"North Korea had best not make any more threats to the United States. They will be met with fire and fury like the world has never seen," Mr Trump said during a briefing on opioid addiction at his golf course in Bedminster, New Jersey.
Japan's defence ministry concluded in an annual white paper released on 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.
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 tho se uncertainties appear to be receding.
Republican senator John McCain said "great leaders" did not threaten foes unless they were ready to act and he was not certain Mr Trump was.
Mr McCain, who heads the Armed Services Committee, made the comments in an interview on KTAR News in Phoenix.
He said he took exception to Mr Trump's comments "because you got to be sure you can do what you say you're going to do".