Russian jets mistakenly bomb US-backed forces in Syria: Pentagon

Russian warplanes bombed US-backed fighters in several small villages in northern Syria after they mistakenly thought Islamic State (IS) group forces were in the area, a US general said on Wednesday.

Army Lieutenant General Stephen Townsend said Russian and Syrian government warplanes attacked the villages to the south and east of al-Bab in Aleppo province on Tuesday, resulting in an unspecified number of casualties.

The Russians apparently had observed Islamic State militants moving from the area and wrongly assumed that other forces remaining were militants.

"We had some Russian aircraft and regime aircraft bomb some villages that I believe they thought were held by ISIS, yet... actually on the ground were some of our Syrian Arab Coalition forces," Townsend told reporters in a video call from Baghdad.

The SAC forms part of a broader alliance called the Syrian Democratic Forces (SDF) comprising Kurdish and other groups that the United States has been training and advising to lead the anti-IS fight.

US troops operating less than five kilometres from the villages observed the bombing and communicated the error, which was then passed to the Russians through a "deconfliction" hotline the two powers have established to avoid mishaps.

"Some quick calls were made to our deconfliction channels and the Russians acknowledged and stopped bombing there," Townsend said.

The Russian defence ministry denied conducting air strikes, but confirmed communication through the hotline.

Townsend also downplayed the chances that the United States would deploy a large number of additional coalition forces to battle Islamic State, even as President Donald Trump weighs options to speed the campaign.

He declined to openly discuss his recommendations for accelerating the fight against IS but his comments represent one of the strongest signals yet that the US military will not advocate any fundamental shift in a key strategy that relies on local ground forces.

The United States now fields less than 6,000 troops in both Iraq and Syria, a far cry from a peak of about 170,000 to Iraq under President George W. Bush.

"I don't foresee us bringing in large numbers of coalition troops, mainly because what we're doing is, in fact, working," Townsend said.

"But in that event that we bring in any additional troops, we'll work that with our local partners, both here in Iraq and Syria, to make sure that they understand the reasons why we're doing that and to get their buy-in of that."