Trump used a speech to conservatives just outside Washington to step up his campaign of "maximum pressure" designed to force North Korea to roll back its weapons programs.

"We imposed today the heaviest sanctions ever imposed on a country before," Trump claimed at the end of a lengthy campaign-style address.

In light of past US embargoes, that is likely an overstatement, but Treasury Secretary Steve Mnuchin confirmed the sanctions covered "virtually all the ships" North Korea is "using at this moment in time."

Trump had been expected to provide details of measures that target "56 vessels, shipping companies, and trade businesses," but skipped that part of his prepared remarks.

"Frankly hopefully something positive can happen," he said instead.

During a joint press conference with Australian Prime Minister Malcolm Turnbull, Trump said that sanctions were only the first action.

"If the sanctions don't work, we'll have to go to phase two. Phase two may be a very rough thing," he said, without elaborating.

Trump's administration is locked in a nuclear standoff with North Korea, which is trying to develop missiles that could deliver an atomic weapon to major US cities.

The latest sanctions are designed to put the squeeze on North Korea's already precarious economy and fuel supply.

Mnuchin said there were signs that the punitive measures were starting to have an impact, but did not elaborate.

The North Korean military and broader economy depend heavily on imports of coal and oil from China and Russia.

China, Pyongyang's only major ally, has steadfastly rebuffed Washington's calls for a full oil embargo -- fearing the chaotic collapse of the Pyongyang regime -- but has agreed to caps agreed at the United Nations.

- Reaffirming ties with Seoul -

The timing of the new measures coincides with the arrival in South Korea of Trump's daughter Ivanka.

She is attending the closing ceremony of the Winter Olympics, which have taken place against the backdrop of the crisis.

The 36-year-old businesswoman and now top aide to her father has been tasked with reaffirming US ties with South Korea, which have been strained over how to deal with the North.

She was hosted in Seoul by President Moon Jae-in, who has long advocated talks rather than confrontation with North Korea.

"Mrs. Trump delivered a personal message to President Moon from President Trump about today's North Korea related sanctions announcement at a small meeting at the Blue House," the White House said.

"They also discussed the continued effort on the joint maximum pressure campaign against North Korea."

South Korean officials, who sit in a capital well within range of conventional North Korea artillery, have been spooked by Trump's easy talk of military confrontation.

US officials worry meanwhile that North Korea is luring Moon into talks that are designed to go nowhere.

Ivanka Trump is not expected to meet North Korean officials on her trip.