Prime Minister Tony Blair has defended the government's decision to dismantle border watchtowers and disband three NI battalions of the Royal Irish Regiment.
He said the moves were "justified" in security terms and had been for a time.
He was speaking after holding separate meetings with DUP leader Ian Paisley and Sinn Fein President Gerry Adams.
Mr Blair said the momentum could only be maintained if people fulfilled their promises to abide by "exclusively peaceful and democratic" means.
The prime minister also said he wanted to see the restoration of devolution as soon as possible, but could not put a timescale on it.
In terms of the security reduction, he said: "These are things that are justified and actually have been justified for some time in security terms.
"With the IRA's statement we can implement that but that has not been forced politically against the security wishes of the police or the Army.
"Obviously you have to mark carefully what happens. You had the IRA's statement but you then have got to make sure that what has been said in theory is carried through in practice."
Ian Paisley said the IRA has done nothing to meet its obligations
Thursday's talks in London between the two party leaders and Prime Minister Blair were the first since last week's IRA statement in which it said it was ending its armed campaign.
After the meeting, the Democratic Unionists said that they would require a "prolonged period of assessment" to determine whether the IRA had given up its armed campaign.
Mr Paisley said the IRA had made a statement but had done nothing about keeping their obligations.
He called for "total decommissioning that everyone can be satisfied with".
Mr Paisley said: "We are not going to have any discussions about devolution until the requirements Mr Blair set out are fulfilled by the IRA," he said.
His deputy, Peter Robinson, added: "It will take a long period of time to make sure that they are gone and they are gone for good."
Mr Paisley said he had presented a list of demands to the prime minister in what he described as a "blunt" meeting.
Mr Adams said he thought devolution could be restored soon
These included assurances relating to the government's announcement that it planned to disband the three home-based battalions of the RIR.
After his meeting, Mr Adams said he thought devolution could be restored soon.
He added he believed that the outstanding elements of the Good Friday Agreement would be implemented "in the period ahead".
Party colleague Martin McGuinness said the DUP had to "regain their nerve" and recognise they had a major contribution to make to the peace process.
"The ball game has changed, changed completely and I think forever," Mr McGuinness said.
"At some stage the DUP will have to respond to an agenda which is very clearly moving on without them."
The Northern Ireland-based battalions of the regiment are to be disbanded on 1 August 2007, as part of the response to the IRA ending its armed campaign.
The Army will end its support role to the police on the same day.
Northern Ireland Secretary of State Peter Hain also announced that troop levels in the province would fall from 10,500 to 5,000 in two years time.