SEOUL (AFP) - North Korea appears to have declined an invitation to send Catholic believers to a mass celebrated by Pope Francis in Seoul later this month, a South Korean Church official said on Tuesday.

In a letter, the North's Korean Catholics Association (KCA) cited Seoul's refusal to cancel an upcoming joint military drill with US forces as the main reason for its decision.

Pope Francis is to conduct a special Korean reconciliation mass in Seoul on the last day of his August 14-18 visit to South Korea, and church officials in the South had sent several requests to Pyongyang to send a group of Catholics to attend.

But the day of the mass coincides with the launch of the annual Ulchi Freedom Guardian joint military drill, which the North had condemned as a rehearsal for nuclear war.

"Under these circumstances, coming to Seoul would be an agonising step," the KCA letter said.

"We interpret this as meaning the North finds it difficult to accept the invitation," said an official at the South Korean Catholic Bishops' Conference.

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 KCA.

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

The KCA claims 3,000 followers in the North but outside estimates put the figure at around 800.

A recent comprehensive report compiled by a UN Commission of Inquiry into human rights in North Korea concluded that practising Christianity outside the state-sanctioned church amounted to a "political crime".

The Pope's final mass, to be held in Seoul's Myeongdong cathedral, will focus on a message of peace and reconciliation for the Korean peninsula.