US Secretary of State John Kerry has met Saudi Arabia's King Salman in Jeddah to discuss the fragile truce in Syria, before taking off for broader talks with Russia, Iran and other countries in Vienna.
Kerry said after the talks on Sunday that he hoped to strengthen a "cessation of hostilities" agreement between Syrian government forces and rebels, which has been undermined by fighting in some areas.
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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.
"On Syria, the secretary provided an update of the situation on the ground following last week's reaffirmation of the cessation of hostilities," a US spokesman said on the meeting, according to the AFP news agency.
"The secretary also gave an update on Libya," he said.
Syria tops agenda
In talks with his Saudi counterpart Adel al-Jubeir, Kerry discussed "regional issues ... mainly developments in Syria," the official Saudi Press Agency reported.
The monarch discussed "aspects of cooperation between the two countries and developments in the region and efforts in that regards," SPA reported.
The US, Saudi Arabia and some other Western and Gulf Arab states plus Turkey back rebels fighting to remove President Bashar al-Assad, who has military support from Russia and Iran.
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However, diplomats in the Gulf say Saudi Arabia sees US 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 Jubeir have previously characterised disagreements over Syria as being limited to "tactical differences" not objectives.
After his talks in Vienna on the Syrian crisis, Kerry will fly on to Brussels on Wednesday for a NATO foreign ministers' meeting and talks on the full range of challenges facing the Western allies.