The sister of Kim Jong Un will make her debut on the world stage at South Korea's Winter Olympics, as the US revealed plans for its "toughest" sanctions yet on the North.

Kim Yo Jong, the youngest daughter of the late Kim Jong Il, will cross the border into the South to attend the opening ceremony on Friday, Seoul officials confirmed.

She is said to be close to her older brother Kim and was promoted last year to become an alternate member of the politburo, North Korea's top decision-making body.

The 30-year-old's visit - the first time any member of the Kim dynasty has visited the country - was announced as the US said it was planning its "toughest and most aggressive" economic sanctions against North Korea in the coming days.

During a visit to Japan, Mike Pence branded North Korea "the most tyrannical and oppressive regime on the planet" as he pushed for the country to abandon its nuclear and missile programmes.

"Over the years, North Korea has…repeatedly supported acts of international terrorism including assassinations on foreign soil," the US vice president said.

"The United States of America will soon unveil the toughest and most aggressive round of economic sanctions on North Korea ever - and we will continue to isolate North Korea until it abandons its nuclear and ballistic missile programs once and for all."

North and South Korea will march under one flag at the Winter Olympics opening ceremony in Pyeongchang.

But Mr Pence warned that the countries had previously "marched under the same banner" at past Olympics "only to see North Korea continue its pursuit of threats and provocations".

A group of 280 North Koreans arrived in South Korea on Wednesday, one of the largest peacetime crossings of the inter-Korean border.

The delegation is made up mostly of a 229-member cheer squad, along with 26 taekwondo performers, 21 journalists and four North Korean Olympics committee members.