BELFAST, Northern Ireland -- Northern Ireland's major Protestant party announced Saturday it will shut down the joint Catholic-Protestant government -- the central achievement of the 1998 peace agreement here -- if the Irish Republican Army doesn't demonstrate it has renounced violence within four months.

First Minister David Trimble, resisting pressure from Ulster Unionist Party hard-liners to pull the plug immediately on a coalition that includes the IRA-linked Sinn Fein party, said he was obliged to set a Jan. 18 deadline and "provide an ultimatum" because his Protestant supporters "are fed up."

Trimble said the Sinn Fein-IRA movement had committed itself to complete "a transition from violence to exclusively peaceful means" as part of its acceptance of the landmark Good Friday pact.

"By the 18th of January, if we don't see that the transition is complete, yes we will be out of office," Trimble said after a combative meeting of the Ulster Unionists' grassroots council that had threatened to end his seven-year reign atop the largest Northern Ireland party.

Trimble committed himself immediately to boycott meetings with the Irish government that would normally involve the two Sinn Fein ministers in his 12-member Cabinet.

He promised to reconvene the 860-member Ulster Unionist Council Jan. 18 to vote on a total party withdrawal from the Cabinet, which requires Ulster Unionist participation to keep operating.

Tensions between the Ulster Unionists and Sinn Fein have frequently undermined the four-party coalition, which was supposed to promote lasting compromise.

Trimble agreed to form the Cabinet in late 1999 on condition that the IRA fully disarmed in response, an as-yet-unfulfilled goal of the peace deal.

The IRA has proceeded slowly with disarmament, scrapping a few arms dumps in secret in October 2001 and April 2002, and has been accused by police of repeatedly breaching its 1997 cease-fire on other fronts.

Perhaps crucially, Trimble declined to specify Saturday what "progress" from the IRA would be sufficient for him to keep propping up power-sharing.

Retired Canadian Gen. John de Chastelain, who for the past five years has overseen efforts to disarm Northern Ireland's myriad outlawed groups, remains on standby to oversee a potential third act of IRA arms decommissioning.