Russian fighters were among those killed when U.S.-led coalition forces clashed with pro-government forces in Syria this month, former associates of the dead said on Monday - however the majority of casualties are believed to be hired mercenaries from para-military company Wagner.

A U.S. official has said more than 100 fighters aligned with Syrian President Bashar al-Assad died when coalition and local coalition-backed forces thwarted a large attack overnight on Feb. 7. However, other local reports place the death toll as high as 200.

Russia's Defence 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.

The Kremlin said on Tuesday it had no information about Russian mercenaries reportedly being killed in Syria, saying it only knew about Russian nationals who had been deployed there as members of the Russian armed forces.

It was responding to media reports citing associates of Russian mercenaries saying some of them had been killed this month when U.S.-led coalition forces clashed with pro-government forces in Syria's Deir al-Zor province.

"We don't have information about other Russians who might be in Syria," Kremlin spokesman Dmitry Peskov told reporters on a conference call when asked about the alleged deaths.

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. Putin's years-long incursion into Syria is deeply unpopular domestically and has long been plagued by reports that the Russian government under reports the official death toll of Russians in Syria for political reasons.

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 Feb. 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 Feb. 7.

The New York Times reported Tuesday that individual Russians have begun speaking out about the death of loved ones in Syria, some are even announcing the specific names of victims on social media.

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.