The prime minister has warned that the government will take a tough line in assessing whether paramilitary groups are observing their ceasefires.

Tony Blair's comments came ahead of a statement by the Northern Ireland secretary, Dr John Reid, in which he said that the government would use the "full force of the law" against terror groups.

Speaking in the Commons, the prime minister said ceasefire judgements should become "increasingly rigorous".

"Now, more than four years after the agreement was signed, it is increasingly urgent that it should be clear that paramilitary organisations are not engaged in any preparations for terrorism and that they should be stood down altogether as soon as possible," he told MPs.

Blair said it was "intolerable that paramilitaries should have played a part in recent sectarian disturbances".

"These disturbances called for and will be met with a strong and effective security response," he said.

"It is no longer sufficient just that there should be no terrorist violence. We have to be clear that preparations for violence have also ceased."

In a Commons statement, Dr Reid warned republicans that terror links abroad and paramilitary violence on the streets of Belfast were incompatible with their ceasefire.

He told MPs that he had written to all the parties in Northern Ireland asking them to reaffirm their commitment to peace.

"Confidence in the process requires confidence there will never again be a return to those dark days," said Dr Reid.

He argued that his regular review of ceasefires must be held "in the round" - but stressed that he would become increasingly "rigorous" in assessing whether any breach had taken place.

The secretary of state had a strong message for those who were undermining the peace process.

"We are willing to spend as long as it takes with those who want to work in partnership with us and with each other," he said.

"Those who do not want to do so should face the full force of the law."

Quentin Davies, the shadow Northern Ireland secretary, said Dr Reid's statement was "vacuous".

He urged the minister to act against terror groups which were in breach of their ceasefire agreements.

Davies warned the government that the last few months had been "a disastrous time for the peace process".

He said Dr Reid now had a "real credibility problem" on the issue of paramilitary violence.

The Lib Dems said the government was right "to lay down the rules" to republican and loyalist paramilitaries.

"People need to understand that violence is the most pointless thing anyone in Northern Ireland can do because they just harm their cause," said the party's Northern Ireland spokesman Lembit Opik ahead of the statement.

"The IRA have been making the right noises recently. They now need to prove their commitment by making the right moves."

Ulster Unionist leader, David Trimble, said the government had to be honest with the people and warned that taking no action against republican groups was no longer an option.