Downing Street said yesterday's IRA withdrawal of its decommissioning plan was "re-grettable" but did not change the fundamental drive toward fully implementing the Belfast Agreement.
The Northern Ireland Secretary, Dr John Reid, said he would not be deflected from the task.
The Conservatives said recent developments cast "serious doubt" on the IRA's commitment to the agreement and the democratic process.
The shadow Northern Ireland Secretary, Mr Andrew Mackay, said the decommissioning of terrorist weapons and a commitment to exclusively democratic and peaceful means were key obligations under the Belfast Agreement.
"Yet, 3 1/2 years on not one gun or one ounce of Semtex has been put beyond use by any of the organisations whose political representatives signed up to the agreement.
"The withdrawal of the IRA of its statement last week, in addition to the arrests of three IRA suspects in Colombia, casts serious doubt on their commitment to both," he said.
Mr Mackay said last week's IRA statement on arms decommissioning did not go far enough and its withdrawal vindicated the cautious response of the Conservative and Ulster Unionist parties to it.
"It also reinforces our view that no further ground should be given on policing and security by the Government. It is essential that paramilitaries - republican and loyalist - fulfil their obligations and actual decommissioning begins.
"Otherwise, the prospect of any progress over the next few weeks remains bleak," he said.
The Liberal Democrat spokesman on Northern Ireland, Mr Lembit Opik, suggested hardline Ulster Unionists were to blame for the current crisis.
"David Trimble has done as much as anyone could to bring his party towards a settlement. The finger of criticism points to hardline Ulster Unionists who have prevented Mr Trimble from making appropriate gestures towards the IRA proposals.
"Of course there is an element of good faith and risk, but the hardliners have not come up with a single better way to normalise the province.
"The onus is now on them to suggest what should happen next," Mr Opik said.