The Russian Defense Ministry said Saturday that US jets had killed more than 60 Syrian soldiers in four airstrikes by two F-16s and two A-10s coming from the direction of Iraq.

The so-called Syrian Observatory for Human Rights, a group based in Britain, cited a military source at Deir ez-Zor airport as saying at least 90 Syrian soldiers had been killed, Reuters reported. 

Russia's Foreign Ministry said in a strongly worded statement that the strikes were "on the boundary between criminal negligence and direct connivance with Daesh (ISIL) terrorists."

It said the incident was a result of Washington's "stubborn refusal" to cooperate with Moscow in fighting Daesh, the Nusra Front – now renamed Jabhat Fatah al-Sham - and "other terrorist groups."