BELFAST, Northern Ireland (AP) _ Assailants shot and killed a former soldier in front of his wife and two children today as a spiral of retaliatory killings continued in Northern Ireland.

No group immediately claimed responsibility for the killing, but suspicion fell on the Irish Republican Army.

The victim, identified by friends as Ronnie Finlay, would be the fourth person killed in sectarian violence in the past week. Finlay had been a part- time soldier for the locally recruited Ulster Defense Force, but had quit over three years ago.

Four years ago, the IRA shot and killed his Finlay's brother, Winston, who was a policeman in the Royal Ulster Constabulary reserve.

On Friday the IRA shot dead Gary Lynch, an election worker for the small Protestant Ulster Democratic Party, at the Londonderry meat plant where he worked.

Saturday, the Ulster Freedom Fighters, an outlawed Protestant paramilitary organization, killed James Carson in the newsagent shop he partly owned in the heart of mainly Catholic west Belfast.

On Monday the Freedom Fighters struck again, killing Patrick Shanaghan, a member of the IRA's legal political wing Sinn Fein, as he drove to work at Castlederg a few miles from the scene of today's killing.

Police said today's victim was shot moments after his wife dropped him off at a farm near Sion Mills in County Tyrone where he worked as a laborer. The victim's wife and children, aged one and nine, watched as gunmen opened fire from the windows of the farmhouse, police said. The three were uninjured.

The gunmen had taken over the farm a short time earlier, tying up the farmer and his family and cutting telephone lines. The killers fled in the farmer's car and abandoned it a few hundred yards from the border with the Republic of Ireland, said a police spokesman, speaking on condition of anonymity.

The Rev. William McCrea, the local member of Parliament for the hard-line Protestant Democratic Unionist Party, called the killing ''vicious and brutal.'' He said it had ''stunned a hard-pressed loyalist community in the border area.''

At Shanaghan's funeral Wednesday, the parish priest said the victim's family requested there be no retaliation.

Ronnie Finlay was the 47th person to die in political and sectarian violence in Northern Ireland this year.

The IRA, supported by a minority of Catholics in Northern Ireland, has attacked police and army units in its effort to end British rule in the province. It also has killed people it claims were Protestant paramilitaries, informers or collaborators with British rule.

Protestant paramilitary groups, rooted in the province's majority Protestant community, have generally attacked people they claim are members of the IRA or other ''republican'' groups opposed to British rule.