International Desk: At least two Russian fighters were killed in US air strikes in north-eastern Syria last week.
The mercenaries are said to have been hired by private military firms backing Syrian pro-government troops.
Russia has not confirmed the deaths that were first reported by US media, saying such reports should not be treated as "primary sources".
The US says it killed more than 100 fighters in the strikes last week.
But Defence Secretary Jim Mattis insisted that despite the media reports he did not have information that "Russian contractors" were among the casualties.
"I can't give you anything on that, we have not received that word at Central Command or the Pentagon," he said.
US officials say hundreds of fighters aligned with Syrian President Bashar al-Assad attacked positions of the US-backed Syrian Democratic Forces (SDF) near the town of Khursham in the Deir al-Zour province.
They crossed the Euphrates River, shelling an SDF base, where American advisers were present.
The US responded with air and artillery strikes on 7 February, thwarting the attack and forcing the pro-Assad fighters to retreat, the US officials say.
risingbd/Feb 14, 2018/Mukul