Sinn Fein and the Democratic Unionists could become Northern Ireland's dominant parties unless the government gets a grip on the political situation, the Ulster Unionist leader has warned.

In an interview with the Guardian on Monday, Northern Ireland First Minister David Trimble said that "nightmare scenario" could take a generation to resolve.

The prime minister has promised to make a statement on the loyalist and republican ceasefires, stressing that there is no acceptable level of paramilitary violence.

It follows demands from unionist politicians for a tougher definition of the ceasefires - what the UUP has called "Mitchell Principles with bite."

If we find ourselves in a situation where the top dogs in Ulster politics are Gerry Adams and Ian Paisley - then it is an absolute disaster

David Trimble First minister

Mr Blair pledged to rebuild confidence in the Northern Ireland process after talks with the pro-Agreement parties and Irish premier Bertie Ahern at Hillsborough Castle, County Down, earlier this month.

Ulster Unionists are demanding that Sinn Fein, as members of the power-sharing executive, be held to account for any breaches of the IRA ceasefire.

'Further gains'

However, Sinn Fein said the crisis was not in the Agreement, but within unionism itself.

Unionists point to allegations that the IRA was behind the break-in at Belfast's police headquarters, during which sensitive Special Branch intelligence was stolen on 17 March.

Tony Blair: Pledged to rebuild confidence in the process

They have also raised allegations of IRA involvement with left-wing guerillas in Colombia and the issue of IRA files detailing information on senior Conservative Party members and Army bases in England.

In the Guardian interview, Mr Trimble said he feared Sinn Fein and the DUP could make further gains in next May's elections to the Northern Ireland Assembly.

He said: "Then we have a crisis, a huge crisis. But if they're both there, if we find ourselves in a situation where the top dogs in Ulster politics are Gerry Adams and Ian Paisley, then it is an absolute disaster."

He said if this happened the entire process would "go pear-shaped".

'Calculated'

The Ulster Unionist leader said Prime Minister Tony Blair must "get a grip on the situation" before mistrust and paramilitarism "pushed Northern Ireland into a political abyss".

He said the government must "not be scared to let republicans know it will take action".

There's still a job to be finished here and it's one I intend to finish

David Trimble

While he did not believe there would be a full-scale resumption of IRA violence, Mr Trimble said allegations of recent IRA activity showed "a very definite strategy of tension and threat".

He described the paramilitary group's apology to its "non-combatant" victims during the Troubles as calculated.

Mr Trimble said he felt his resignations as first minister on two previous occasions had been positive, but said he was determined not to quit from his own Ulster Unionist Party.

"There's still a job to be finished here and it's one I intend to finish," he said.

He said his controversial proposal for a border poll on whether Northern Ireland should remain part of the United Kingdom was the best way to settle the dispute.

The first minister said this should take place on the same day as the assembly elections.