U.S. Secretary of State John Kerry has met Saudi Arabia’s King Salman in Jeddah. The two discussed the fragile truce in Syria, just two days before the Vienna talks.

Kerry said he hopes to strengthen a “cessation of hostilities” agreement between Syrian government forces and rebels. The aim is to increase humanitarian aid deliveries to besieged areas.

The U.S., Saudi Arabia, and some other Western and Gulf states, plus Turkey, back rebels fighting to remove President Bashar al-Assad. But diplomats in the Gulf say Saudi Arabia sees U.S. support for the rebels as inadequate. They say Riyadh fears that Washington may abandon their shared stance that Assad must immediately leave power as part of any negotiated political deal.

Kerry and his Saudi counterpart Adel al-Jubeir have previously characterized disagreements over Syria as being limited to “tactical differences,” not objectives.