US Vice President Mike Pence vowed tough new sanctions against North Korea and called it the world's most tyrannical regime yesterday, two days before he is due to attend the winter Olympics, along with two of the North's most senior officials.

Speaking in Tokyo on his way to South Korea, which is hosting the Games over the next three weeks, Pence said he would soon announce the stepped-up sanctions in an effort to pressure the North to abandon its nuclear and missile programmes.

The Games, being staged 80 km from the heavily militarised border between the two Koreas, is set for an awkward political encounter, with Pence as well as the North Korean leader's younger sister attending Friday's opening ceremony.

South Korea wants to use the event to re-engage with the North and pave the way for talks to resolve a political crisis widely regarded as one of the world's most dangerous, with US President Donald Trump and Pyongyang swapping nuclear threats.

"I'm announcing today the United States of America will soon unveil the toughest and most aggressive round of economic sanctions on North Korea ever," Pence said after meeting Japanese Prime Minister Shinzo Abe.

"We will continue to isolate North Korea until it abandons its nuclear and ballistic missile program once and for all."

Washington is taking a much tougher line on the North than its allies in Seoul, exposing tensions that South Korean President Moon Jae-in could struggle to conceal at the Olympics.

Pence has voiced scepticism that the North, which is participating in the Games, will use the event for crude propaganda. As his guest at the opening ceremony, he is taking the father of Otto Warmbier, an American student who died last year after being imprisoned in North Korea for 17 months.

Sitting in the same stadium as VIP guests will be Kim Yo Jong, 28-year-old sister of North Korean leader Kim Jong Un, and Kim Yong Nam, the North's nominal head of state.

Pence has not ruled out the prospect of meeting North Korean officials during the Olympics but President Trump has cast doubt on US negotiations with Pyongyang any time soon.

"We will not allow North Korea to hide behind the Olympic banner the reality that they enslave their people and threaten the wider region," Pence said.

Also yesterday, a group of 280 North Koreans arrived in South Korea, one of the largest peacetime crossings of the inter-Korean border, to spur on athletes from the two sides at the Winter Olympics.

The group included a 229-member cheer squad as well as taekwondo performers, journalists and four North Korean Olympics committee members, including the sports minister.

On Tuesday, a ferry carrying a North Korean orchestra arrived for the Games. Since it docked, North Korea has asked the South to provide oil for refuelling.