LONDON, OCT. 24 -- The Irish Republican Army used "proxy" car bombs to kill six soldiers and a civilian and wound more than two dozen people in attacks on security checkpoints in Northern Ireland today.

The attacks, which took place simultaneously before dawn at two checkpoints at opposite ends of the province, caused the highest one-day death toll of soldiers in Northern Ireland in more than two years. Analysts said they illustrated the IRA's ability to adjust tactics and strike in a coordinated fashion.

Masked gunmen seized Catholic families in three locations in Northern Ireland Tuesday night and held them hostage at gunpoint, forcing the heads of the households to drive their own vehicles, loaded with explosives, to the targets, according to the police.

Outside Londonderry in the northwest corner of the province, one of the huge bombs killed five soldiers and one unidentified civilian, probably the driver, police said. The blast seriously wounded six other soldiers and police and slightly injured 11 civilians living nearby.

Witnesses said the blast virtually obliterated the checkpoint, leaving a deep crater and smoldering rubble. Peter Wiley, who lives nearby, said he heard a massive explosion that shattered windows and damaged roofs throughout the area. "It was an unbelievable bang," he told BBC Radio. "The whole checkpoint is devastated. . . . There's nothing left."

In the second blast, near Newry, on the main Belfast-Dublin road in southeast Northern Ireland, the unidentified driver leaped from his vehicle before it exploded and shouted, "There is a bomb in the van." The bomb went off seconds later, killing one soldier and wounding 10. The driver, a 65-year-old retiree, escaped with a broken leg.

An army base near Omagh in the center of the province was the target of a third attack, but the 200-pound bomb failed to detonate.

The IRA claimed responsibility for the attacks in a statement to reporters in Belfast. "This morning's military operation again devastates the myth of containment," it said. "Until the British government end their futile war in Ireland, attacks such as this morning's will continue."

The statement charged that the civilians forced to drive the bombs were "collaborators" who had done contruction work for the security forces.

"Proxy bombs" have been used frequently in the past by the IRA to deliver explosives. But today's attacks were believed to be the first time the civilian proxies were not given a chance to escape before the bombs went off.

Seamus Mallon, a member of Parliament for the moderate and predominantly Catholic Social Democratic and Labor Party, said the Catholic community generally would condemn the use of such proxies.

"To use a 65-year-old man to do the IRA's dirty work is the essence of cowardice," he said. "I think there will be great anxiety about the use of proxy bombs in this way because it means that every single person who owns a car in the north of Ireland is vulnerable, every single person can be used in that way," he said.

Peter Brooke, the British cabinet secretary in charge of Northern Ireland, told reporters: "These wanton murders and the abuse of civilians as hostages demonstrate the inhumanity of the terrorists' methods and the sterility of their thinking."

Police in the Irish Republic arrested five men in County Donegal for questioning about the Londonderry attack. The bombing is expected to set off new calls for tougher border restrictions between the province and the republic, and may make harder Brooke's faltering attempt to arrange talks between nationalists and unionists in Northern Ireland.

Two weeks ago, commandos in a British special forces unit shot dead two top IRA gunmen, Dessie Grew and Martin McCaughey, in an undercover operation. Some observers suggested today's attacks were meant as retaliation.

The bloodiest attack on the army in Northern Ireland in recent years came in August 1988 when eight soldiers were killed and 19 injured when a bomb blew up their bus.