President Bush and Mr Blair will meet NI parties later on Tuesday
President George Bush says he will urge Northern Ireland politicians to "seize the opportunity for peace" during a crucial week for the province's political process.
The President was speaking after holding talks on the war in Iraq with Prime Minister Tony Blair.
The two leaders have been joined by Taoiseach Bertie Ahern and will see the pro-Agreement parties later on Tuesday to discuss the stalled Northern Ireland political process.
Northern Ireland's power-sharing administration was suspended on 14 October 2002 amid allegations of IRA intelligence gathering at the heart of the Stormont government.
President Bush said: "Later this week, Prime Minister Blair and the taoiseach will release a plan setting out the remaining actions that must be taken to realise the promise of the Good Friday Agreement.
"I support and my government strongly supports their efforts.
It is both significant and welcome that the president is here in person to join in urging the parties to take the final steps towards a lasting peace
Prime Minister Tony Blair
"At the meeting this afternoon I will urge Northern Ireland's political leaders to adopt this plan as their own.
"This is a historic moment and I ask all the communities of Northern Ireland to seize this opportunity for peace."
Mr Blair said it was "both significant and welcome that the president is here in person to join in urging the parties to take the final steps towards a lasting peace".
"Here we are, for all the difficulties in Northern Ireland, able to point back to real improvements in security and the standard of living of people here, and point forward to turning progress into lasting change, lasting security and lasting peace, which is want people want to see here.
US NORTHERN IRELAND POLICY
We view paramilitaries and the sectarianism that allows them to thrive as the chief obstacle to normalisation
US special envoy Richard Haass
"We have made that progress because of patience and perseverance and because friends like those in the USA have helped us get there."
It is expected Mr Blair and Mr Ahern will return on Thursday - the fifth anniversary of the Good Friday Agreement - to unveil their proposals which they hope could move the process forward.
Later on Tuesday, a working lunch will be followed by round-table talks with the pro-Agreement parties.
Speaking before meeting President Bush, Sinn Fein President Gerry Adams said that "no deal has been done".
He said: "Issues like sanctions, the whole range of matters around demilitarisation and the transfer of powers on justice and policing remain critical and crucial elements."
Meanwhile, Ulster Unionist leader David Trimble described Mr Bush's words as positive and supportive.
He said the president had given a strong signal to republicans that time was running out.
Bertie Ahern (centre) joined the leaders for a working lunch
The village of Hillsborough in County Down has been virtually sealed off amid intense security for Mr Bush's two-day visit.
Ahead of the presidential visit, Secretary of State Paul Murphy said President Bush could help persuade the IRA to disarm.
Meanwhile, anti-Agreement Democratic Unionist MP Gregory Campbell said their voters were being discriminated against because the party had not been invited to the talks.
The party was offered a meeting with Ambassador Richard Haass, the US special envoy to Northern Ireland, but declined it.
Last month, Mr Blair and Mr Ahern held talks in the province, but failed to reach agreement with the pro-Agreement parties on a number of key issues.
Unionists insist on sanctions against parties who break the terms of the Agreement, however Sinn Fein object to the measures which they say are aimed solely at them.
Other controversial proposals by the governments to restore the Northern Ireland Assembly include allowing "on-the-run" paramilitaries to be dealt with through a judicial commission.
With an IRA statement expected some time after the joint statement by the
British and Irish premiers, Mr Trimble is demanding that disarmament has to be
both visible and transparent.