Pentagon chief Ashton Carter on Friday hit out at Russia for bombing US-backed forces in southern Syria who he said were fighting the Islamic State group, calling their actions "problematic."

"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, using an alternate acronym for the IS group.

Carter admitted that perhaps Russian forces made a mistake when they conducted a series of air strikes on Thursday near al-Tanaf — but said in that case, it highlighted poor intelligence on their side.

"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."

On Thursday, a senior US defense official said that Russian aircraft had not been active in the al-Tanaf area on Syria's border with Iraq "for some time" and that there were "no Syrian regime or Russian ground forces in the vicinity."

The Britain-based Syrian Observatory for Human Rights said two US-backed fighters — one Syrian and one Iraqi — were killed in the strikes.

The Syrian belonged a group of fighters from the New Syrian Army, trained by the British and the Americans in a coalition camp in Jordan, while the Iraqi was a tribal fighter, Observatory chief Rami Abdel Rahman said.

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

The United States and Russia have put in place a channel of communication so that they can ensure that their separate air campaigns in Syria do not result in any unsafe incidents.

But Carter said the communications link "was not professionally used."

"We are trying to clarify the facts and use that channel with the Russians to understand what went on," he said.

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 known as the 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.

Two US-backed fighters killed in Syria raids: monitor
Beirut (AFP) June 17, 2016 –

Two US-backed fighters opposed to the Islamic State 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 jihadists 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-jihadist rebels in support of President Bashar al-Assad's regime.

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