US fighter jets have fired warning flares in Syrian airspace after Russian fighter planes entered a non-conflict area, the US defence department said.
The Pentagon said two Russian Su-25s flew into an area east of the Euphrates River on Wednesday when the two US jets sent the warnings.
One of the two F-22 US jets was almost involved in a mid-air collision with a Russian plane during the incident which lasted “several minutes”, one American official said.
Russia has disputed the US report. The Russian defence ministry said in a statement its jets had been escorting a humanitarian convoy near a town by the Euphrates river when the US planes approached.
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The Pentagon said that calls had been made "on the emergency channel to convey to the Russian pilots that they needed to depart the area".
US forces operate on the east side of the Euphrates River, while Russia operate on the west side as part of a verbal agreement between the two countries.
However, the US said Russian planes were frequently flying into its zone to the east of the river.
"It's become increasingly tough for our pilots to discern whether Russian pilots' actions are deliberate or if these are just honest mistakes," a US defence department spokesman said.
Lt Col Damien Pickart, a spokesman for the US Air Forces Central Command, warned Russians planes crossing the dividing line could end in aircraft being shot down.
"The greatest concern is that we could shoot down a Russian aircraft because its actions are seen as a threat to our ground forces," he told CNN last week.
"As coalition leaders have repeatedly said, we're here to fight Isis -- not the Russians or anyone else."
