The UN mediator for Syria says he will hold talks with Russian Deputy Foreign Minister Gennady Gatilov in Geneva on Monday and that the US has declined to take part in any trilateral meetings for now.
Israel on Wednesday said it believes Syrian regime forces still possess several tonnes of chemical weapons.
The announcement comes two weeks after a deadly chemical attack in Syria's Idlib which killed nearly 90 people.
Israel, along with many countries, blames the strike on regime forces. French Foreign Minister Jean-Marc Ayrault said French intelligence services would provide proof of that in the coming days.
A senior Israeli military officer, in a briefing to Israeli reporters, said "a few tonnes of chemical weapons" remain in the hands of Bashar al Assad's regime forces.
Some local media reports quoted the briefing officer, who spoke on condition of anonymity in line with Israeli military procedure, as putting the amount at up to 3 tonnes.
In a 2013 agreement brokered by Russia and the United States, Syria agreed to destroy its chemical weapons.
Earlier in the day, the Organisation for the Prohibition of Chemical Weapons, a global watchdog, said sarin or a similarly banned toxin was used in the April 4 strike in Syria's Idlib province.
The findings supported earlier testing by Turkish and British laboratories.
Israeli Defence Minister Avigdor Lieberman told Israel's Yedioth Ahronoth newspaper on April 6 that he was "100 percent certain" that the attack was "directly ordered and planned by Assad." He did not elaborate on how he had reached that conclusion.
UN Envoy, Trilateral Talks
The UN mediator for Syria said on Thursday he would hold talks with Russian Deputy Foreign Minister Gennady Gatilov in Geneva on Monday and that the United States had declined to take part in any trilateral meetings for now.
"The trilateral meeting that as you know was a possibility is being postponed, it is not taking place on Monday. It will be a bilateral. But the trilateral is not off the table, just being postponed," UN Special Envoy to Syria Staffan de Mistura told reporters.
De Mistura, asked about the US administration intent to participate, replied: "There is a clearly an intention to maintain and resume these trilateral discussions, the date and circumstances were not conducive for this to happen on Monday."