Tony Blair said there could be "no twin track of politics and violence".

Speaking at the second of his "presidential-style" media briefings on Thursday, Mr Blair said there must be a return to "first principles" over ceasefires and participation in Northern Ireland's government.

While recognising loyalist violence, Mr Blair said it was Sinn Fein and the republican movement which was sitting in government.

Sinn Fein are in government and it is quite clear that you cannot carry on in government unless you are committed to exclusively peaceful means

Tony Blair

"We are looking carefully, as John Reid said yesterday, at precisely how to give credibility to any assessment. That we have some mechanism to assist us in this process," he said.

He added: "We have to be able as a government to make a judgement necessarily more rigorous as time goes on that if these activities continue it is inconsistent with people being on a ceasefire."

Mr Blair there was "no doubt at all" that if the ceasefire was breached the secretary of state would put a motion before the Northern Ireland Assembly.

He added: "Sinn Fein are in government and it is quite clear that you cannot carry on in government unless you are committed to exclusively peaceful means.

The prime minister said peace settlements could lead to "messy compromises" and grey areas.

However, he said the idea that unionists had got nothing from the Good Friday Agreement was "absurd".

Meanwhile, the Ulster Unionist leader is meeting his assembly party to discuss the implications of Wednesday's Commons statement by the prime minister on the Northern Ireland ceasefires.

David Trimble had asked for the statement, but was disappointed the government did not take a tougher line with paramilitaries.

Mr Trimble will use Thursday afternoon's meeting to assess the current mood within his assembly group. It has backed him in the past.

In the Commons statement, Mr Blair said there would be a rigorous response to breaches of loyalist and republican ceasefires.

Tony Blair outlined details in Commons

But he stopped short of outlining any new sanctions against parties linked to paramilitary groups.

The statement was in response to pressure from unionists for Sinn Fein's exclusion from the Stormont executive for what they claim are breaches of the IRA ceasefire.

Mr Trimble is keeping his options open. It may be September until he makes his final decision. In the meantime, attempts may be made by hardliners to try to force his hand.

They want him to withdraw the party from government, forcing the collapse of the assembly.

Paramilitary violence

Meanwhile, Sinn Fein President Gerry Adams is meeting the Taoiseach, Bertie Ahern, in Dublin on Thursday.

The government's assessment came against a backdrop of recent trouble in Belfast and the sectarian killing of a teenager which was admitted by the loyalist paramilitary Ulster Freedom Fighters.

David Trimble: Government should have gone further

Northern Ireland Secretary John Reid said he was considering using an assessment of levels of paramilitary violence within both loyalist and republican communities to supplement his decisions on ceasefires and would decide on this over the summer.

However, Shadow Secretary of State Quentin Davies has called for stiffer sanctions against organisations in breach of their ceasefires.