WASHINGTON - The Pentagon voiced "strong concerns" over Moscow's bombing of US-backed forces in southern Syria, a spokesman said on Saturday.

US military officials holding a video conference with Russia "expressed strong concerns about the attack on the coalition-supported counter-ISIL forces at the Al-Tanaf garrison, which included forces that are participants in the cessation of hostilities in Syria," said Pentagon spokesman Peter Cook.

He added that the Pentagon "emphasised that those concerns would be addressed through ongoing diplomatic discussions." US defense officials said that Russian forces in Al-Tanaf targeted a meeting of combatants supported by the US-led coalition that was held to coordinate the fight against ISIS.

The Syrians belonged to the New Syrian Army, trained by the British and the Americans while the Iraqis were tribal fighters, officials said.

Russia's defense ministry said late Thursday however that it had not carried out any strikes targeting opposition forces included in the ceasefire, without mentioning Al-Tanaf -- an assertion challenged by the United States.

"Regarding safety, department officials conveyed that Russia's continued strikes at Al-Tanaf, even after US attempts to inform Russian forces through proper channels," Cook said.

Russian defence chief, Assad coordinate on fight

Russian Defence Minister Sergei Shoigu met in Damascus on Saturday with President Bashar al-Assad to discuss anti-terrorist "cooperation" in Syria's conflict, a bone of contention between Moscow and Washington.

Shoigu, the highest ranking Russian official to travel to Syria since the conflict erupted five years ago, was sent by President Vladimir Putin for the surprise meeting with Moscow's long-time ally Assad, the Syrian news agency SANA said.

"The talks focused on military cooperation between the two countries and joint action to fight against terrorist organisations on Syrian soil," it said.

Shoigu, whose country's military intervention since last September turned the tide of the conflict in favour of Assad's regime, also inspected Russia's air base in Hmeimim in the Syrian coastal province of Latakia, his ministry said.