Russian fighters were among those killed when US-led coalition forces clashed with pro-government forces in Syria this month, former associates of the dead said on Monday.
A US official has said more than 100 fighters aligned with Syrian President Bashar Assad died when coalition and local coalition-backed forces thwarted a large attack overnight on February 7.
Russia's Defense Ministry, which supports Assad's forces in the Syrian civil war, said at the time that pro-government militias involved in the incident had been carrying out reconnaissance and no Russian servicemen had been in the area.
But at least two Russian men fighting informally with pro-government forces were killed in the incident in Deir al-Zor province, their associates told Reuters on Monday.
One of the dead was named as Vladimir Loginov, a Cossack from Russia's Kaliningrad exclave. Maxim Buga, a leader of the Cossack community there, said Loginov had been killed around February 7 along with "dozens" of other Russian fighters.
The other man killed was named as Kirill Ananiev, described as a radical Russian nationalist. Alexander Averin, a spokesman for the nationalist party he was linked to, told Reuters Ananiev had been killed in shelling in the same fighting on February 7.
Reuters was unable to independently confirm either man's death.
Grigory Yavlinsky, a veteran liberal politician who is running for president in elections next month, called on President Vladimir Putin to disclose how many Russians had been killed in Syria and in what circumstances.
"If there was large-scale loss of life of Russian citizens, the relevant officials, including the commander-in-chief of our armed forces (Putin), are obliged to tell the country about it and decide who carries responsibility for this," Yavlinsky said in a statement released by his Yabloko party.