Nicolas Sarkozy, the French president, has told staff to suspend diplomatic contacts with Syria until Paris has proof that Damascus is working for a consensus president in Lebanon.
Speaking in Egypt after talks with Hosni Mubarak, the Egyptian president, Sarkozy said the time had come for deeds rather than words from Syria.
"We cannot wait any longer. Syria must stop talking, must demonstrate," he said.
"I will not make any more contacts with Syria ... [and] all of my colleagues ... as long as there is no proof of Syria's will to let Lebanon choose a consensus president," he added.
Lebanon has not had a president since November 23 due to disagreements between the ruling coalition and the opposition over the country's political orientation.
Consensus candidate
Rival leaders have agreed on army chief General Michel Suleiman as a consensus candidate to be president but they are still wrangling over how to share power once he takes office.
The conflict reflects a regional struggle for influence between Syria and Iran on one side, and the United States and its European and Arab allies on the other.
France played a lead role in mediating the agreement on Suleiman's candidacy and has been frustrated that the deal has not yet been completed.
Mubarak told the same news conference in Egypt that it was unacceptable that Lebanon should go for months without a president and this could lead to unspecified complications.
He appealed to Syria to use its influence in Lebanon to make sure parliament elects a new president.
"I ask Syria to intervene with the influence that it basically has in Lebanon to work to create agreement," he said.