ARMAGH, NORTHERN IRELAND, JULY 24 -- A 1,000-pound bomb exploded under a rural road near this border city today, killing three policemen and a Roman Catholic nun, authorities said.

An anonymous caller claimed responsibility for the attack on behalf of the Irish Republican Army in North Armagh in a telephone call to a news organization. The caller said the IRA had killed the four people in a "military action," expressing no regret over the nun's death.

The slaying of a nun could be a major embarrassment for the mainly Catholic IRA, which in recent months has escalated its terror campaign aimed at driving the British out of Northern Ireland.

It was believed to be the first time a Catholic nun has been killed in 21 years of sectarian violence in British-run Northern Ireland, according to Jim Cantwell of the Catholic Press and Information Office.

"I know all the people of Ireland join me in my condemnation of this atrocity," Prime Minister Charles Haughey of the Irish Republic said.

The bomb set in a culvert under the road was detonated as an unmarked police car passed a hayfield two miles outside Armagh, police said. The nun and a female social worker were approaching in a small car, police said.

Sister Catherine Dunne died later in a hospital. Her companion, whose name was not released, was listed in satisfactory condition, police said.

The Royal Ulster Constabulary officers were identified as William Hanson, 37, David Sterritt, 34, and Joshua Willis, 35.

Police and troops rushed reinforcements to search the border area, which is considered prime territory for operations by the IRA. In Belfast, a police spokesman said an unknown number of gunmen took over a house about 400 yards from the road early today and held a married couple and their children at gunpoint until shortly before the 2 p.m. attack.

Police said a detonating wire ran from the bomb through a freshly cut hayfield to a spot near the house.

The bombing was the deadliest attack in Northern Ireland since an IRA bomb killed four soldiers of the locally recruited Ulster Defense Regiment on April 9. It follows a spate of attacks by the IRA on British targets in England and continental Europe.

Archbishop Robin Eames, the Anglican primate of Ireland, said, "My reaction is one of absolute horror that somebody . . . fulfilling their own calling as a nun has been murdered in what is one more indiscriminate attack on the Royal Ulster Constabulary."

Nearly 3,000 people have been killed since sectarian violence erupted in Northern Ireland in 1969.