US and Russian aircraft have conducted communication tests over Syria, US officials said, in tentative signs the powers are working together ahead of UN talks aimed at ending the war.
Russian officials on Tuesday also said its jets bombed 24 targets in Syria using coordinates supplied by "opposition representatives", the first time it has claimed to work with those fighting Syria's government since beginning its air offensive.
"The coordinates of all of these targets were given to us by opposition representatives," senior military official Andrei Kartapolov said, without specifying the groups involved.
Hours later the Pentagon said its fighter pilots communicated directly with Russian jets in the skies over Syria, in the first test of a new strategy to ensure the two sides' parallel campaigns do not boil over into conflict between them.
Washington and Moscow signed an agreement on October 20, laying out rules to keep their pilots away from each other in the air, after several close encounters raised the prospect of a midair collision or some other dangerous encounter.
A US-led coalition has been bombing Islamic State of Iraq and the Levant (ISIL) forces since December 2014, while the Russians opened an air campaign in September against a broader range of rebels that Western powers say is designed to support its ally, Syrian President Bashar al-Assad.
The burgeoning coordination between the two comes as Russian Foreign Minister Sergey Lavrov is set to meet the United Nations' Syria envoy Staffan de Mistura in Moscow on Wednesday.
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The two met Friday in Vienna alongside top diplomats from 17 other international players including the United States, Iran, and Saudi Arabia, in the broadest push yet to end the four-year conflict.
The participants - who did not include any representatives of the Syrian government or its opponents - agreed to ask the UN to broker a peace deal between the regime and opposition to clear the way for a new constitution and elections.
But divisions remain on the fate of Assad, with Russia and Iran resisting pressure from Western powers and Saudi Arabia to force the Syrian leader from power.
Syria's Deputy Foreign Minister Faisal Moqdad on Tuesday ruled out any transition period in the war-torn country, insisting that Assad is "the legitimate president elected by the Syrian people".
Russia offered on Tuesday to host a meeting between representatives of the Syrian government and rebel groups next week in Moscow, and said it has given Saudi Arabia and the US a list of the opposition figures it is working with.
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Moscow said it had set up "working coordination groups" aimed at bolstering the fight against ISIL, but that the identities of those involved were being kept secret.
"Such close cooperation will allow us to unite the efforts of the government troops with other patriotic forces in Syria that used to be in the opposition and act as a united front against the common enemy - international terrorism," the defence ministry said.
The latest strikes hit targets close to Palmyra, Deir Ezzor, Ithriya and eastern Aleppo with assistance from the opposition, destroying command posts, munition stores and anti-aircraft artillery, it added.
Moscow has been bombing targets in Syria since September 30, and said on Tuesday it has hit 2,084 targets in 1,631 sorties, including 52 training camps and 287 command posts, causing "significant losses to the terrorists" and undermining their morale.
"Our aim both in Syria and anywhere else is to fight terrorism first of all," Russian President Vladimir Putin said in Moscow.
The Britain-based Syrian Observatory for Human Rights said warplanes belonging to either Russia or the Syrian government bombed ISIL's de facto capital, Raqqa, on Tuesday, killing at least 23 people including 13 fighters.