A senior US military official said on Monday it was a Russian airstrike - not a US-led coalition hit - that killed four Syrian soldiers this weekend, the Associated Press has reported.

The soldiers were killed and 13 others injured when a bomb hit an army camp in eastern Syria province of Deir Ezzor on Sunday, the UK-based Syrian Observatory for Human Rights reported earlier on Monday.

The observatory said the air raid was conducted "by the international coalition" and was the first time that the coalition had killed Syrian government troops.

The US-led coalition, however, denied it was involved in the reported attack. Hours later, an unnamed US military official - who reportedly spoke on condition on anonymity because he was not authorised to discuss the matter publicly - has said that the country is "certain" that Russia conducted the airstrike, AP reported.

Pentagon spokesman Capt Jeff Davis, however, later said that while Russia flew long-range bomber missions into Syria on Sunday, the US has "not been able to correlate one to the other," the BBC reported.

Syria's foreign ministry described the attack as a "flagrant aggression", saying it "blatantly violates the objectives of the UN charter".

Neither the observatory, nor the Syrian government named which member of the coalition, which now includes the UK and France, they believe carried out the raid.

The US-led coalition has been targeting IS in Syria since September last year, expanding a campaign that began with raids in neighbouring Iraq.

Its operations have expanded further in recent days, partly in response to the deadly attacks in Paris claimed by IS.

On Sunday, its jets hit IS positions in Raqqa province, killing at least 49 people, mainly IS militants, according to the Observatory. It said that eight children and five women were killed in the raids.

The monitor relies on a network of activists, medical staff and fighters on the ground who identify jets based on model, flight patterns and munition types.

Syria's conflict has killed more than 250,000 people since March 2011, and another four million have been forced to flee the country.