Pentagon 'concerned' by Russian bombing of US-backed Syrian rebels

The Pentagon has 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 Russian counterparts "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, using another acronym for Islamic State.

He added that the Pentagon "emphasized that those concerns would be addressed through ongoing diplomatic discussions on the cessation of hostilities."

US defence 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 IS fighters in Syria and Iraq.

Russia's defence 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 of on-going coalition air support to the counter-ISIL forces, created safety concerns for US and coalition forces," Cook said.

"(Defence) Department officials requested Russian responses to address those concerns. The two sides reiterated the need to adhere to measures to enhance operational safety and avoid accidents and misunderstandings in the air space over Syria."

Pentagon chief Ashton Carter on Friday slammed the Russian bombing, stressing that the forces that were attacked had been anti-IS troops.

"This was an attack on forces, first of all, that were fighting ISIL. Obviously that's the first thing that's problematic about this Russian conduct," Carter told reporters.

"If it was their intention, it is the opposite of what they said they were going to do," he said. "If not, it says something about the quality of information upon which they make their strikes."

The US military launched a $500 million program in early 2015 to train entire units of "moderate" Syrians to fight Islamic State jihadists.

But the program drew heavy fire last fall after admitting the efforts had floundered, with numbers of trainees falling massively short of the planned 5,000.

One group even handed over ammunition and other gear to a local al-Qaeda affiliate, al-Nusra Front.

Since then, the Pentagon's new strategy is to work with just a handful of members from each fighting group, instead of an entire unit.

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

Agencies contributed to this report.