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According to a defence ministry statement, the two Russians were making a test flight in the Homs region with a Syrian Mi-25 helicopter that was carrying ammunition.

It said Isil fighters broke through Syrian forces' lines east of Palmyra at the same time.

The statement said the Syrians requested the crew help strike the Isil fighters and the helicopter was shot down after the crew exhausted the aircraft's ammunition and were leaving the scene.

An Mi-24 Russian helicopter. credit:

The United States is reportedly seeking to establish a new military alliance with Russia in Syria that would see the two countries join forces in combating jihadist groups.

The proposal, which is said to have been sent to Moscow this week, would mark a striking change in stated US policy on Syria, with the two super powers having operated largely on opposing sides of the war between the government and anti-regime rebels.