"Diplomatically and militarily, Moscow plays both arsonist and firefighter, fueling tensions among all parties in Syria," General Joe Votel, who heads the military's Central Command (Centcom), told lawmakers.

Moscow then serves "as an arbiter to resolve disputes, attempting to undermine and weaken each party's bargaining positions."

Russian President Vladimir Putin stepped into Syria's civil war in late 2015, when the Russian military began assisting the regime of President Bashar al-Assad.

More than two years later, with momentum on Assad's side, Putin ordered the partial withdrawal of Russia's troops from Syria in December, saying their task in the war-torn country had been largely completed.

Moscow is pursuing a peace process in the Kazakh capital Astana -- separate from a United Nations-led process in Geneva -- along with Turkey and Iran.

"Moscow continues to advocate for alternate diplomatic initiatives to Western-led political negotiations in Syria and Afghan-led peace processes in Afghanistan, attempting to thwart the UN's role and limit the advance of American influence," Votel told the House Armed Services Committee.

"Either Russia has to admit it is not capable or it does not want to play a role in ending the Syrian conflict. I think their role is incredibly destabilizing at this point," he added.

Votel also charged that Russia is using its presence in Syria to test and exercise new weapons and tactics, "often with little regard for collateral damage or civilian casualties."

In Syria, the US-led coalition is backing a largely Kurdish group called the Syrian Democratic Forces, which are fighting the Islamic State group.

The coalition on February 7 struck a formation of pro-regime forces attacking an SDF position east of the Euphrates river in eastern Syria, killing about 100 people.

Many Russian mercenaries were reported killed in the strike, according to their relatives and paramilitary groups, as well as political organizations that published information about the casualties.