WASHINGTON, Aug 26 (Reuters) - The U.S.'s new special envoy for Syria, Michael Ratney, will visit Moscow, Riyadh and Geneva later this week for discussions on trying to find a political solution to the four-year Syrian conflict.

A senior State Department official said Ratney, who was named as the new envoy on July 27, will travel to the three capitals from Aug. 28 to Sept. 2.

"Special Envoy Ratney will meet August 28 in Moscow with senior Russian officials and August 29 in Riyadh with senior Saudi officials to continue discussions about working towards a genuine political transition and bringing an end to the devastating crisis in Syria," the official said.

In Geneva, he will meet with the UN's special envoy for Syria, Staffan de Mistura.

The Obama administration has long insisted that any diplomatic effort has to include the departure of Syrian President Bashar al-Assad, who is increasingly under military threat from an Islamic State insurgency.

The U.S. has welcomed initiatives by Russia, an ally of Assad's, and the U.N. to find a diplomatic solution to the crisis although insisting that any transfer of power must be within the so-called Geneva framework, which calls for the transfer of power to a transitional governing body.

Assad said in an interview broadcast on Tuesday he was open to an idea of a coalition against Islamic State but indicated there was little change of it happening with his enemies, referring to Turkey and Saudi Arabia that have backed insurgent groups fighting to topple him.

Assad's comments cast further doubt on a Russian plan to forge an alliance against the group. Saudi Arabia has ruled out any coalition with Assad and like the United States wants to see Assad gone.

(Reporting by Lesley Wroughton; Editing by Bernard Orr)

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