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 May 13, 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.

On May 17 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.