Donald Trump has said North Korea "will be met with fire and fury like the world has never seen" if it escalates its nuclear threat, after US media reported the dictatorship had produced a miniaturised warhead.
On Tuesday, The Washington Post published extracts from a Defence Intelligence Agency report which details North Korea's success in producing nuclear missiles.
"The IC [intelligence community] assesses North Korea has produced nuclear weapons for ballistic missile delivery, to include delivery by ICBM-class missiles," the assessment states, in an excerpt read to the newspaper.
ICBM, or inter-continental ballistic missiles, have been tested by North Korea over recent weeks, prompting fresh sanctions from the United Nations.
North Korea threatened "thousands-fold" revenge for the sanctions.
In response Trump, who was at his New Jersey golf course, told reporters on Tuesday: "North Korea best not make any more threats to the United States.
"They will be met with fire and fury like the world has never seen," he told reporters.
Trump said of North Korean dictator Kim Kong-un: "He has been very threatening ― beyond a normal statement.
"As I said, they will be met with fire, fury and frankly power the likes of which this world has never seen before."
The ICBMs could potentially reach 10,000km - the reported capability of missiles tested by North Korea in July and which landed in the Sea of Japan.
That distance would bring the warheads within reach of the US east coast, including New York and Washington DC.
The US pushed for sanctions against North Korea on Saturday after its latest round of tests, citing them as a contravention of UN resolutions.
The Security Council unanimously passed a resolution banning North Korean exports of coal, iron, iron ore, lead, lead ore and seafood, intending to press the Asian state to renounce its nuclear weapons and ballistic missile programs.
Trump has previously pressed for more action against the country.
The leaked document backs up an increased estimate of North Korea's nuclear stockpile. That estimate found leader Kim Jong Un controls up to 60 nuclear weapons.
Crucially, however, the document links the state's nuclear capability and its ICBM programme - something military experts had said would take the state years longer to realise.
Five fully-fledged nuclear states are part of the UN treaty on non-proliferation of nuclear weapons: Britain, the United States, France, China, and Russia.
Other states which say they have nuclear weapons include North Korea, Pakistan, and India.
Israel has refused to talk in detail about its possible nuclear programme.
Other countries including Turkey, Italy and Germany have stored nuclear weapons on behalf of the United States.