US President Bush's special adviser on Northern Ireland tonight condemned Mr David Trimble's description of the Irish Republic as a "pathetic sectarian state".
Ambassador Mr Richard Haass said the Ulster Unionist leader's statement to his party's annual general meeting was regrettable.
Mr Haass, speaking on BBC Northern Ireland's Hearts and Minds programme said: "That's exactly the sort of language that ought to be avoided.
"We ought not to be talking in ways that reinforce or sharpen sectarian lines. We ought to be reaching out."
Mr Trimble, who is in Washington for the St Patrick's celebrations, has refused to withdraw his remarks.
He caused further uproar when he claimed the recent abortion referendum in the Republic was sectarian. Mr Haass repeated his assertion that political leaders in Northern Ireland were failing to reach out to the entire community.
"Clearly the leadership of Northern Ireland is not where I think it should be.
"What I would like to do is see leadership increasingly preparing people for the notion that they have got a common future," he said.
Admitting that he was baffled by the continued opposition of many unionists to the Belfast Agreement, Mr Haass appealed to unionist leaders to try to sell the peace process.
"The situation is better than it was a year ago. It is better than it was five or 10 years ago, so I find it hard to understand how political leaders can make the case persuasively that things are not better," he added.
Meanwhile, the cross community Alliance Party tonight strongly opposed Mr Trimble's call for a referendum on whether there should be a united Ireland on the day of the next Northern Ireland Assembly elections.
Speaking after a delegation met Secretary of State Dr John Reid in Washington, Alliance leader Mr David Ford claimed:
"A border poll (the referendum) is the most polarising, divisive and destabilising issue that could be put to the people of Northern Ireland."
Mr Ford said the timing of a referendum on the day of a major election would be particularly damaging.
"The Secretary of State should only move to call a poll if there are clear suggestions that a change in Northern Ireland's constitutional status is likely," he added.
PA