US imposes 'heaviest' sanctions

US President Donald Trump rolled out sanctions against North Korea-linked shipping assets Friday, hailing the package as the "heaviest sanctions ever" levied on the Pyongyang regime.

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 90 minute 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's administration is locked in a nuclear standoff with North Korea, which is trying to develop missiles that could deliver a nuke to major cities in the United States.

The tougher sanctions may jeopardise the latest detente between the two Koreas, illustrated by the North's participation in the Winter Olympics in the South, amid preparations for talks about a possible summit between North Korean leader Kim Jong Un and South Korean President Moon Jae-in.

Kim said he wants to boost the "warm climate of reconciliation and dialogue" with South Korea after a high-level delegation, including his sister, returned from the Olympics.

In December the United Nations approved US-drafted limiting North Korea's access to refined petroleum products and crude oil, which the North Korean Foreign Ministry said amounted to an act of war. In January Washington announced a round of sanctions and urged China and Russia to expel North Koreans raising funds for the programmes.

The new US sanctions were announced while Trump's daughter, Ivanka, is visiting South Korea. She had dinner with Moon after a closed-door meeting with the president.

At a dinner with Moon at Seoul's presidential Blue House, she said the purpose was also to "reaffirm our commitment to our maximum pressure campaign to ensure that the Korean Peninsula is denuclearised."

Moon, cited by his spokesman, Yoon Young-chan, said talks with the North on denuclearisation and improving inter-Korean relations must go hand in hand.

Ivanka's visit to South Korea coincides with that of a sanctioned North Korean official, Kim Yong Chol, blamed for the deadly 2010 sinking of a South Korean navy ship that killed 46 sailors. His delegation will attend the closing ceremony and also meet Moon.

North Korea has denied any involvement.

Seoul said it approved the pending visit by Kim Yong Chol in the pursuit of peace and asked for public understanding in the face of opposition protests.

South Korea's decision on Thursday to allow in Kim Yong Chol, currently sanctioned by the United States and South Korea, sparked protest from family members of the dead sailors and opposition parties.

North Korea last year conducted dozens of missile launches and its sixth and largest nuclear test in defiance of UN Security Council resolutions. It defends the weapons programmes as essential to deter US aggression.