The French Foreign Minister, Herve de Charette, said in London on Friday that if the men were not handed back by midnight yesterday, the entire Bosnian peace process could be in danger. France gave no indication of what it might do in retaliation.
Last week, President Jacques Chirac threatened the Bosnian Serbs with unspecified consequences if the pilots were not produced, and at last week's Peace Implementation Conference in London, France made clear that prospects for peace in former Yugoslavia could hinge on the pilots' fate.
Paris is keen to ensure that there is clarity about the men before it hosts the ceremonial signing of the Dayton accords in Paris on Thursday. Pressure built yesterday for postponement of the ceremony.
"It's out of the question to sign the Bosnia peace treaty when our husbands have not been returned," Isabelle Souvignet, wife of the missing navigator Jose Souvignet, told the newspaper Journal de Dimanche. The paper said it would be impossible for Mr Chirac to appear alongside President Slobodan Milosevic of Serbia, who negotatiated on behalf of the Bosnian Serbs.
The magazine Paris Match published photographs in September that purported to show the two pilots, Frederic Chiffot and Mr Souvignet, still alive after their Mirage jet was shot down over the Bosnian Serb "capital", Pale, on 30 August. Early last week, the French Defence Minister, Charles Millon, said he had information that the two were being held by a Bosnian Serb group.
But Mr Chirac said later he had no reliable information even that the pilots were alive. A French parliamentary delegation went to Belgrade last week to find out more, but left with no further information.
The most bizarre contribution was a statement two months ago by Mr Milosevic that the two pilots had been removed from a hospital in Pale and their whereabouts was no longer known.
The statement was interpreted variously as strange enough to be true, or a fabrication to cover up the fact that Mr Milosevic had been unable to produce them.
Last week, one French report said the two had been seriously injured while in captivity - it was speculated that one had lost a leg - and that this was the reason both why they had been in hospital and why they had not yet been handed back.
At the same time, Radovan Karadzic, the Bosnian Serb leader, said that Mr Chirac could speed up freedom for the pilots if he obtained assurances for Bosnian Serbs over their future in Sarajevo.
Officially, France expressed disgust that the safety of the pilots could be treated as a bargaining chip. Since the accords were agreed in Dayton, however, France has argued for improved guarantees for Bosnian Serbs in Sarajevo.
It was impossible to say whether Mr Karadzic's veiled threat meant the pilots were still alive or whether it was a last, cynical attempt to benefit from the continued uncertainty.