There will be a rigorous response to breaches of loyalist and republican ceasefires, the prime minister has said.

Tony Blair and the Northern Ireland secretary set out new conditions in the House of Commons on Wednesday to judge whether the paramilitaries were holding to their ceasefires.

The prime minister said paramilitary organisations should be stood down while the development of weapons and targeting of individuals should also cease.

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

Tony Blair aiming to rebuild confidence

He told MPs that four years after the Good Friday Agreement it was "no longer sufficient that there should be no terrorist violence - we have to be clear that preparations for violence have ceased".

"It is right that with the passage of time these judgments should become increasingly rigorous."

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.

The government's assessment comes 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.

Later in the Commons, NI 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.

David Trimble: Government should have gone further

He revealed the government was considering extending police powers in a bid to clamp down on paramilitary activity.

"I have asked the Attorney General to lead an examination of police powers, bail arrangements and the scope for additional criminal offences.

Ulster Unionist leader David Trimble told the Commons the government should have gone further.

He said a decision on whether ceasefires had been breached should not be left to the secretary of state.

Sinn Fein said an opportunity to confront the crisis in the peace process was missed.

Party leader Gerry Adams said: "The British Government has failed to deal with the real crisis which our society is facing and that is the sustained and orchestrated campaign of sectarian violence by loyalist groups."

Shadow NI spokesman Quentin Davies said a tougher response to violence was needed, and claimed Dr Reid's speech was full of "vacuousness".

John Reid: Police powers to be examined

The DUP's Peter Robinson dismissed Mr Blair's statement.

He said people were being asked to believe the word of a "serial promise-breaker".

SDLP leader Mark Durkan said unionists must give explicit support to the devolved institutions if they wanted an end to paramilitary violence.

Irish Prime Minister Bertie Ahern said he was confident that Mr Reid would continue to perform the task of making assessments of the terrorist ceasefires "in the round and in accordance with the relevant statutory requirement".

Mr Blair pledged to rebuild confidence in the Northern Ireland process after talks with the pro-Agreement parties and Irish premier Bertie Ahern at Hillsborough Castle, County Down, earlier this month.

Unionists point to allegations that the IRA was behind the break-in at Belfast's police headquarters, during which sensitive Special Branch intelligence was stolen on 17 March.

They have also raised allegations of IRA involvement with left-wing guerrillas in Colombia and the issue of IRA files detailing information on senior Conservative Party members and Army bases in England.