Mr Bush will meet the pro-Agreement party leaders at Hillsborough
President George Bush could help persuade the IRA to disarm, the Northern Ireland Secretary, Paul Murphy, has said.
Mr Bush arrives in the province on Monday for talks with Prime Minister Tony Blair on the war in Iraq.
Mr Blair arrived at Hillsborough Castle at about 1715 BST.
President Bush is expected to be accompanied by US Secretary of State Colin Powell, and Richard Haass, the special American envoy on Northern Ireland.
Mr Murphy will greet the delegation at Aldergrove, before accompanying them by helicopter to Hillsborough Castle for a war summit with Mr Blair and Foreign Secretary Jack Straw.
The village of Hillsborough in County Down has been virtually sealed off amid intense security.
Both leaders are also expected to discuss the political process in Northern Ireland and are to meet the leaders of the pro-Agreement parties and Irish Taoiseach Bertie Ahern at Hillsborough Castle.
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.
The talks are expected to continue on Tuesday.
The secretary of state said the summit would be extremely helpful for the Northern Ireland peace process.
"I do hope that in the next week we will see these acts of completion being described, whether it is from us in terms of our joint declaration, or whether it is the IRA doing what they have to do," he said.
"I think the fact that we have got these world leaders and prime ministers with us will actually help that process."
Speaking on Monday morning, Ulster Unionist Party leader David Trimble said this week was "the moment of truth" for the political process.
Mr Trimble said he would be disappointed if the outstanding issues in the process were not resolved by Thursday.
The anti-Agreement Democratic Unionist Party has expressed outrage at not being invited to give its views to Mr Bush.
The party was offered a meeting with Ambassador Haass.
However, party leader Ian Paisley said: "Given the willingness to accommodate a meeting between the President and those who have a smaller mandate than the Democratic Unionist Party, we will not be meeting with Ambassador Haass at this time."
With Mr Blair and Mr Ahern due to return to the province on Thursday to reveal their blueprint for restoring devolved government, many see Mr Bush's arrival as being instrumental in delivering a final breakthrough.
Mr Blair and Mr Ahern held crucial talks in the province last month, but failed to reach agreement with the pro-Agreement parties on a number of key issues.
Mr Adams criticised the decision to hold the summit in NI
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 paramilitary "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.
Sinn Fein President Gerry Adams said his party needed to be reassured that there would be no
further threats from unionists to bring down the power-sharing institutions.
On Sunday, Mr Adams said the decision to hold what he called a "war summit" in Northern Ireland was insensitive.
SDLP leader Mark Durkan, who will meet President Bush on Tuesday, encouraged his party supporters to take part in peaceful, dignified protests against the war in Iraq.