North Korea fired five short-range rockets into the sea off its east coast Thursday, just as Pope Francis arrived in Seoul for a five-day visit.

The launches began at 9:30 am (0030 GMT) at a site near the North's eastern port of Wonsan, with the rockets fired into the East Sea (Sea of Japan) at a maximum range of 220 kilometres (130 miles), a defence ministry spokesman said.

Three were fired in the morning and two in the afternoon, he said.

"They are presumed to have been fired from a 300-millimetre multiple rocket launcher," he said, adding that the military had stepped up vigilance along the heavily fortified border.

The pope is expected to send a message of peace to Pyongyang when he conducts a special inter-Korean "reconciliation" mass in Seoul next week 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.

Thursday's launches came hours after North Korea warned that if South Korea failed to cancel an upcoming military drill with the United States it would push the two sides "to the brink of war".

In a statement that offered no direct response to Seoul's recent offer of high-level talks, the Committee for the Peaceful Reunification of Korea, which handles cross-border ties, issued a long list of measures the South should implement if it was "sincere" about improving relations.

The joint military drill scheduled to begin Monday "should be cancelled unconditionally", the statement said.

The annual Ulchi-Freedom Guardian exercise is aimed at testing combat readiness for a North Korean invasion.

Although largely played out on computers, it involves tens of thousands of South Korean and US troops.

North Korea has carried out an extended series of missile tests into the East Sea in recent months, despite UN resolutions barring it from any launches using ballistic missile technology.

The North has defended the tests as a legitimate exercise in self-defence and a response to the South-US war manoeuvres.