North Korea fired three short-range rockets into the Sea of Japan just before Pope Francis arrived in Seoul on his first visit to Asia.

The Pope will spend five days in South Korea, meeting some of the country's five million Catholics on the first papal trip to the country in 25 years.

The last rocket was fired 35 minutes before Pope Francis was due to touch down.

"North Korea fired three short-range projectiles into the East Sea (Sea of Japan)," a South Korean defence ministry spokesman said.

Pope Francis is expected to send a message of peace to Pyongyang when he conducts a special inter-Korean "reconciliation" mass in Seoul on the last day of his visit.

Church officials in the South had sent several requests to Pyongyang to send a group of Catholics to attend the event, but the North declined the offer, citing its anger at upcoming South Korea-US military drills.

The Catholic Church, like any other religion, is only allowed to operate in North Korea under extremely tight restrictions, and within the confines of the state-controlled Korean Catholics Association.

It has no hierarchical links with the Vatican and there are no known Catholic priests or nuns.

Message to China

The Pope has also sent unprecedented message of good will to China as he flew over the country that does not allow its Catholics to recognise his authority.

"Upon entering Chinese air space, I extend best wishes to your Excellency and your fellow citizens and I invoke the divine blessing of peace and well-being upon the nation," he said in a radio message to President Xi Jinping.

It was the first time a pope had been allowed to fly over China on Asian tours.

His predecessor John Paul II had to avoid Chinese airspace when visiting Asia, because of the fraught relations between Beijing and the Vatican.

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The Vatican has had no formal relations with China since shortly after the Communist Party took power in 1949.

The Catholic Church in China is divided into two communities: an "official" Church known as the "Patriotic Association" answerable to the Party, and an underground Church that swears allegiance only to the pope in Rome.

The Pope did not mention China or the divided Korean peninsula in his brief chat with reporters on the plane, but instead insisted on the media's role in promoting peace in a divided and violent world.

"May your words help unite ... I ask you to always give a message of peace, always seek a message of peace because what is happening (in the world) now is ugly," he said.

Pope meets Sewol families

The Pope was greeted at an air base in southern Seoul by South Korean president Park Geun-hye with a delegation of South Korean Catholics including two North Korean defectors.

Also present were relatives of victims of the Sewol ferry that capsized in April, killing about 300 passengers, most of them school children.

"The disaster is heart-breaking, I have not forgotten the victims," Pope Francis said.

The pope was scheduled to attend a formal welcoming ceremony later in the day, in order to allow the 77-year-old pontiff several hours for rest after the long flight.

Pope Francis has on previous foreign visits opted to ditch his bulletproof 'Popemobile' car in favour of more modest modes of transportation.

In Seoul, he was picked up at the end of the red carpet in a dark grey Kia Soul hatchback.

The visit, the main purpose of which is for the pope to preside at a gathering of Asian Catholic youth, is the third international trip by Francis since his election in March 2013.

ABC/Reuters