"Today, Russian aircraft conducted a series of air strikes near al-Tanf against Syrian Counter-ISIL forces that included individuals who have received US support," said the senior defence official yesterday, who requested anonymity.
"Russian aircraft have not been active in this area of southern Syria for some time, and there were no Syrian regime or Russian ground forces in the vicinity," the official said.
It was not known how many fighters were struck and the extent of casualties or which group they belonged to.
One group even handed over ammunition and other gear to a local Al-Qaeda affiliate, known as the Al-Nusra Front.
Since then, the Pentagon's new strategy is to work with just a handful of members from each fighting group, instead of an entire unit. Much of the attention is being focused on the Syrian Democratic Forces, a largely Kurdish coalition that has scored some significant gains against IS jihadists.
The bombing would likely further strain already testy ties between Moscow and Washington on the Syrian issue.
"Russia's latest actions raise serious concern about Russian intentions. We will seek an explanation from Russia on why it took this action and assurances this will not happen again," the defence official said.
Russia and the United States co-chair a 22-nation group that supports a UN-led process to end Syria's five-year civil war through a negotiated deal.
On Wednesday, US Secretary of State John Kerry told Russia and Syrian President Bashar al-Assad to respect a fragile ceasefire, warning that Washington's patience was running out.
The US has accused Russia of working to consolidate the regime of Assad, its ally, and continuing to attack the opposition.
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