The two mediators said they had spent long enough clarifying their plans to Radovan Karadzic, the Bosnian Serb leader. A spokesman said their demand was not an ultimatum, but he added that the co-chairmen saw no point in dragging out the negotiations any further.
The issues at stake were Serbian reservations over the sovereignty of the Bosnian state and the rights and powers of its proposed autonomous provinces.
The move came as Mr Karadzic appeared in tortuous fashion to be modifying his stance and it represented a classic diplomatic gambit to bring him to the brink of a decision.
It ended a day of drawn-out and confused meetings in hotel suites and conference rooms. The Serbian President, Slobodan Milosevic, self-cast in the role of Balkan peacemaker, joined the talks for the first time as his allies adopted a newly conciliatory tone.
Mr Milosevic made a regal progression between meetings with the Bosnian Serbs at the five-star Hotel des Bergues on Lake Geneva and lunch later with Lord Owen and Mr Vance at the United Nations.
The two mediators witnessed a well-choreographed performance in which Mr Milosevic promised his good offices for peace while Mr Karadzic uttered a series of ever-more-conciliatory, if less than consistent, statements.
'As you know we are here to support peace,' the Serbian President said at the United Nations. 'Our hope is that all sides will use this opportunity to stop this cruel war and to make peace prevail in Bosnia-Herzegovina.'
Mr Milosevic said the Owen-Vance represented a good basis for a settlement, but he insisted that the rights of Serbs, Muslims and Croats must be granted equal respect.
For his part, Mr Karadzic told the BBC that 'we do not want secession'. But he qualified this, adding that he 'could not accept that we do not have the right to make various relationships and agreements with neighbouring or any other countries either'.
It was evidently this sort of ambiguity that the co-chairmen were determined to resolve. Their 10-point draft constitution would forbid the provinces from making treaties with foreign powers.
Criticism of the Owen-Vance plan has been building in the United States and both men are eager to avoid the taint of appeasement. 'The sovereignty of Bosnia-Herzegovina is not negotiable,' their spokesman said.
Mr Karadzic has given a personal assurance to British officials that he holds British troops in the highest regard and his men have no orders to attack them.
He was responding verbally to a formal protest over the shelling of a British base at Tomislavgrad in south-west Bosnia. The protest was lodged by a senior British diplomat late on Sunday night on the instructions of the Foreign Secretary, Douglas Hurd.
Mr Karadzic replied that he did not believe that any forces under what he described as the unified military command of the Bosnian Serbs had carried out the attack. He suggested it could have been the work of individuals whose relatives had been killed by Muslim or Croat forces.
It is understood that further contacts are likely between British officials and Mr Karadzic.Reuse content