Mr David Trimble said today Mr Tony Blair must make a decision soon on whether to restore devolved institutions in Northern Ireland if the elections to the Assembly are to go ahead.
Speaking after an hour-long meeting with Mr Blair in London, Mr Trimble said it was "increasingly problematic" whether the elections scheduled for May can go ahead.
It was questionable whether there was any point holding an election to an institution which was suspended, he said.
Mr Trimble this morning asked the Sinn Féin leadership for talks on what steps the republican movement might make to end the crisis in the peace process.
He said the UUP wanted to see not only acts of decommissioning by the IRA, but also assurances that the organisation will disband and paramilitaries will end "all violent and illegal activities".
Speaking after what he said was "a fairly frank and useful discussion" with Mr Blair, Mr Trimble said: "The [British] government will have to take a decision n this and will have to take a decision fairly soon.
"Our preference would be for matters to be resolved in such a way that the institutions could be re-formed and then proceed to go to an election, but whether that is possible in the time available is now a very open question.
Mr Blair is expected to meet the SDLP on Thursday and Sinn Féin later this week.
Sinn Féin's Gerry Kelly yesterday dismissed calls for his party to be excluded from devolved government.