Two US-backed fighters opposed to the Islamic State (IS) group were killed in air strikes in Syria that Washington has blamed on Moscow, a monitor said Friday.

One Syrian and one Iraqi died in Thursday's raids on a camp in Homs province near the border with Iraq, the Syrian Observatory for Human Rights said.

The strikes in Al-Tanaf targeted a meeting of combatants supported by the US-led coalition that was held "to coordinate the fight against IS in Syria and Iraq", Observatory chief Rami Abdel Rahman said.

The Syrians belonged to the New Syrian Army, trained by the British and the Americans in a coalition camp in Jordan, while the Iraqis were tribal fighters, he said.

The coalition is supporting twin offensives in Syria and Iraq against IS Militants who declared a cross-border "caliphate" in 2014.

A senior US defence official in Washington earlier accused Russia of bombing US-backed fighters in Al-Tanaf and said the incident raised "serious concern".

"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.

The Observatory said it was not the first time that Russia had targeted US-backed rebels since launching an aerial campaign in support of Syria's regime in September.

The Britain-based monitor -- which relies on a network of sources inside Syria for its information -- says it determines what planes carried out raids according to their type, location, flight patterns and the munitions involved.

Moscow says that it targets "terrorists" in its raids, but it has been accused of hitting non-Militant rebels in support of President Bashar al-Assad's regime.

Moscow and Washington are sponsors of a nationwide ceasefire between the regime and non-Militant rebels implemented in February that hangs by a thread.

Russia's defence ministry said late Thursday that it had not carried out any strikes targeting opposition forces included in the ceasefire, without mentioning Al-Tanaf.

Syria's five-year war has killed more than 280,000 people and displaced millions.

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