The US and NI voters "are fast losing patience" with the inability of the parties and governments to restore devolution, Sir Reg Empey has said.
Sir Reg Empey was speaking in Washington
The Ulster Unionist leader said a lack of unionist confidence in republicans "should not leave us paralysed".
"If we do not nail down a deal soon, we risk an unravelling of our newly found stability with all the implications that would flow from that," he said.
Sir Reg was speaking at the National Press Club in Washington.
Devolved government at Stormont was suspended in October 2002 following allegations of a republican spy ring at the Northern Ireland Office.
Sir Reg asked: "What to do now? Can we go home from this St Patrick's celebration and form a fully inclusive executive upon our return? I'm afraid not.
"Due to the level of criminality identified in the January report of the Independent Monitoring Commission, the backwash from the Northern Bank robbery and the McCartney affair, unionist confidence in republicans has taken a big knock.
"But that should not leave us paralysed. We must move on in a way that will build up trust and confidence.
"I am convinced that when Prime Minister Blair and Taoiseach Ahern produce their 'Roadmap' in a few weeks, it must inject real momentum into our politics."
Sir Reg said the UUP was prepared to consider the proposals of all parties and "we have advanced our own model for an interim solution if full agreement is not reached".
He and other Northern Ireland party leaders are expected to attend a reception in the White House on Friday.