Saudi Arabia's King Salman (R) meets U.S. Secretary of State John Kerry in Jeddah, Saudi Arabia May 15, 2016. Saudi Press Agency/Handout via REUTERS

JEDDAH (Reuters) - U.S. Secretary of State John Kerry met Saudi Arabia’s King Salman in Jeddah on Sunday to discuss the fragile truce in Syria, before broader talks with Russia, Iran and other countries in Vienna on Tuesday.

Kerry has said he hopes to strengthen a “cessation of hostilities” agreement between Syrian government forces and rebels, which has been undermined by fighting in some areas, and to increase humanitarian aid deliveries to besieged areas.

On Friday, he said the meetings with the king and the Saudi interior and defense ministers - the two most senior princes - would try “to make sure that we can get this cessation better footed and, frankly, better observed and implemented throughout the country.”

The United States, Saudi Arabia and some other Western and Gulf states plus Turkey back rebels fighting to remove President Bashar al-Assad, who has military support from Russia and Iran.

However, diplomats in the Gulf say Saudi Arabia sees U.S. support for the rebels as inadequate, and 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.

On Tuesday the United States and Russia will co-chair a meeting of the International Syria Support Group, which includes Arab League and European Union countries as well as Turkey, Iran and China.