The Russian military has denied claims that it struck a US-backed force in eastern Syria, wounding six fighters.
The Kurdish-led and US-supported Syrian Democratic Forces said its fighters were hit in the air strike near the eastern city of Deir el-Zour in an industrial area that was recently liberated from the so-called 'Islamic State' group.
Western forces with the SDF were not injured, the US military said.
The SDF is supported by a US-led international coalition of forces to defeat IS militants in Syria and Iraq.
An estimated 900 US troops are embedded with partner forces in Syria. They provide artillery support and can command air support.
The Russian defence ministry said: "Russian air forces carry out pinpoint strikes only on IS targets that have been observed and confirmed through several channels."
SDF fighters have been advancing against 'IS' fighters on the east bank of the Euphrates while Syrian government forces and their allies are pushing on the western side against the jihadists.
The march by the SDF aims to prevent Syrian troops and their allies from expanding their presence along the border with Iraq.
Also today, the UN's World Food Program halted its air drops to Deir el-Zour after its trucks were to reach the city with food relief, for the first time since May 2014.
A five truck convoy brought with it enough wheat to feed 70,000 people, the organisation said in a statement. Monitoring groups reported that residents were receiving wheat distributions the same day.
With the city besieged by militants from the 'Islamic State' group, the WFP began delivering aid through high-altitude air drops in April last year. It flew missions five times a week and completed 309 air drops before halting the program. Nearly 100,000 people were trapped under the siege.
Pro-government forces broke the siege on September 5 and secured the road to the capital, Damascus, shortly after. It immediately began organising its own aid deliveries to the city.
The government now controls two-thirds of Deir el-Zour, according to the Britain-based Syrian Observatory for Human Rights monitoring group.