GENEVA (Reuters) - U.S. Secretary of State John Kerry and Russian Foreign Minister Sergei Lavrov opened talks on Thursday on disarming Syria's chemical weapons programs, but differences emerged at the outset of the expected two-day negotiations. Kerry reiterated the U.S. position that military force might be needed against Syria if diplomacy over President Bashar al-Assad's chemical weapons stockpile fails. "President (Barack) Obama has made clear that should diplomacy fail force might be necessary to deter and degrade Assad's capacity to deliver these weapons," Kerry said, as Lavrov looked on. But Lavrov made it clear that Russia wants the United States to set aside its military threats for now. As the U.S. Congress debated military strikes as a response to an August 21 chemical attack on a suburb of Damascus, Russia proposed that Syria instead agree to give up its chemical arms. Kerry made clear that Washington, while exploring the offer, remains skeptical. And he pushed back on a reported offer from the Syrian government, as part of a move to join the Chemical Weapons Convention, to supply data on its chemical arsenal within 30 days, the standard practice. "We believe there is nothing standard about this process at this moment because of the way the regime has behaved, not only the existence of these weapons but they have been used," Kerry said. Additional reporting by Tom Miles; Editing by Sonya Hepinstall