Two people whose families allegedlt were being held hostage by the IRA drove car bombs up to British checkpoints Wednesday, causing explosions that killed six soldiers, wounded 21 people and left one driver missing.

Both blasts occurred at 4:13 a.m. on the eve of the anniversary of a pact giving Dublin a greater say on Northern Ireland, police said, and involved people forced to drive the car bombs because their families were being held hostage.The worst attack, in Londonderry about 75 miles northwest of Belfast, killed five soldiers and wounded eight other people. The driver of the car was missing and presumed dead. The other attack at Nawry, about 40 miles south of Belfast, killed one soldier and wounded 13 other people.

The Irish Republican Army in a telephone call to a local radio station claimed responsibility for the attacks --the most devastating in two years -- and said the civilian were chosen because they were in the construction industry and had worked for the security forces.

The actions by the radicals fighting to end British rule in Northern Ireland came on the eve of the fifth anniversary of the signing of the Anglo-Irish agreement, a pact between the British and Irish governments giving Dublin a greater say in the affairs of Northern Ireland.

In London, Prime Minister Margaret Thatcher expressed her shock at the killings and declared her sympathies for the families of the victims.

Peter Brooke, the secretary of state for Northern Ireland, said, "These wanton murders and the abuse of civilians as hostages demonstrated the inhumanity of terrorist methods and the sterility of their thinking."