"If previously we had suspicions that Al-Nusra Front is protected this way, now, after today's airstrikes on the Syrian army we come to a really terrifying conclusion for the entire world: The White House is defending the IS," Russian Foreign Ministry spokeswoman Maria Zakharova told Rossiya 24.
"We demand a full and detailed explanation from Washington. That explanation must be given at the UN Security Council," Zakharova added.
The Foreign Ministry has demanded full and detailed explanation from Washington over the incident in Deir al-Zor, in which 62 Syrian troops were killed and over 100 injured.
Moscow stressed that it is concerned by the US airstrike targeting the Syrian army units which had been destroying the Islamic State militants.
The ministry added that the Russian side had never been notified of US plans to carry out bombings in the Deir al-Zor area.
The Russian Foreign Ministry stressed that US air strikes against Syrian government forces jeopardize the Syrian ceasefire deal reached between Moscow and Washington on September 9.
Earlier on Saturday Russia's Defense Ministry said that four strikes against Syrian positions had been delivered by US-led coalition aircraft - two F-16 jet fighters and two A-10 support aircraft.
The Syrian military called the bombing a "serious and blatant aggression" against Syrian forces, and said it was "conclusive evidence" that the US and its allies support IS militants.
US Central Command later issued a statement, saying that the US-led "coalition would not intentionally strike a known Syrian military unit." It promised to look into the incident.
