North Korean leader Kim Jong Un's sister joined athletes and officials from around the world at the South's Winter Olympics on Friday, the first member of Pyongyang's ruling dynasty to set foot in its rival since the Korean War.

Kim Yo Jong was part of a diplomatic delegation led by ceremonial head of state Kim Yong Nam — the highest-level North Korean official ever to go to the South — as the Games trigger a diplomatic rapprochement between the rivals.

Ahead of the opening ceremony, the South's President Moon Jae-in shook hands with Kim Yong Nam at a leaders' gathering in Pyeongchang, but Seoul's Blue House said US Vice President Mike Pence did not and left before the event ended.

Both Washington and Tokyo are regularly threatened by Pyongyang, but Japanese Prime Minister Shinzo Abe did shake hands and exchange words with Kim, Seoul's spokesman said.

Moon is scheduled to have lunch with the Pyongyang delegation on Saturday.

Their white Ilyushin-62 jet, marked in Korean script "Democratic People's Republic of Korea", the North's official name, and its tailfin emblazoned with a Northern emblem, touched down earlier at Incheon airport near Seoul, in a rare direct flight between the two halves of the divided peninsula.

The last member of the Kim family to set foot in Seoul was Yo Jong's grandfather Kim Il Sung, the North's founder, after his forces invaded in 1950 and the capital fell.

Three years later the conflict ended with a ceasefire rather than a peace treaty, leaving the two sides technically in a state of war.

Now the North is subject to multiple rounds of UN Security Council sanctions over its banned nuclear weapons and ballistic missile programmes, while the democratic South has risen to become the world's 11th-largest economy.

Kim Yong Nam and Kim Yo Jong, both of them in dark coats with fur collars, were met at the airport by the South's unification minister and other officials, exchanging pleasantries about the cold weather.

The leader's sister looked relaxed, smiling calmly as she talked with them, before making her way through the terminal, with four bodyguards surrounding her closely, to take a high-speed train to Pyeongchang.

The delegation's trip is the diplomatic high point of a Games-driven rapprochement between the two Koreas, with the dovish Moon pushing a "peace Olympics" that will open a door for dialogue to alleviate tensions and seek to persuade Pyongyang to give up its atomic ambitions.

But all eyes are on Yo Jong — a key member of the Kim dynasty that has ruled the impoverished, isolated nation with an iron fist and pervasive personality cult over three generations.

The family are revered in the North as the "Paektu bloodline", named after the country's highest mountain and supposed birthplace of the late leader Kim Jong Il.

Many analysts suggest Yo Jong may be carrying a personal message to Moon from her brother.

'Charm offensive'

Tensions have been high on the peninsula since last year when the North staged its sixth and most powerful nuclear blast and test-fired intercontinental ballistic missiles (ICBMS) capable of reaching the US mainland.

Leader Kim and US President Donald Trump exchanged threats of war and personal insults, sparking global alarm and fears of a new conflict on the peninsula.

But Kim abruptly announced a plan to send athletes and high-level delegates to the Pyeongchang Winter Games in his new year speech, setting in motion a flurry of cross-border talks and trips.

The announcement — following months of cajoling by Seoul — is seen as a bid to defuse tensions and seek a loosening of the sanctions against it.

The North has sent a total of 22 athletes plus hundreds of cheerleaders and artistes for the Olympics and a state orchestra gave one of two planned concerts in the South on Thursday to a packed audience.

But Pyongyang also held a military parade the same day, displaying its hulking ICBMs in Kim Il Sung Square in a show of strength, and the diplomatic manoeuvres have met a backlash in the South, with many accusing Seoul of making too many concessions to its wayward neighbour.

Conservative activists also accused Pyongyang of "hijacking" the Games and have held angry protests, burning images of Kim Jong Un or the North's national flag.

US Vice President Pence — who has not ruled out a meeting with the North's delegates — on Friday called Pyongyang "the most tyrannical regime on the planet" as he met defectors at a memorial to the Cheonan, a South Korean corvette that sank in 2010, killing 46 sailors.

An international investigation concluded it had been torpedoed by a North Korean submarine, a charge Pyongyang denies.

Pence intended to counter "what Prime Minister Abe rightly called a 'charm offensive' around the Olympics" by the North, he said.

His objective was "to stand up for the truth", he said, "and to recognise that whatever images may emerge against the powerful backdrop and idealism of the Olympics, North Korea has to accept change."