US Syria strikes 'killed up to 200 Russian fighters' in Deir az-Zour
Russian President Vladimir Putin's spokesman, Dmitry Peskov, declined to comment on reports of Russian casualties, saying the Kremlin only tracks data on the country's armed forces.
Putin spoke with US President Donald Trump by phone on Monday, but the military action in Syria wasn't discussed, he said.
The attack occurred on 7 February when pro-Assad forces fired artillery rounds and advanced in a "battalion-sized formation supported by artillery, tanks, multiple-launch rocket systems and mortars," Colonel Thomas F. Veale, a spokesman for the US military, said in a statement.
US military forces responded with aircraft and artillery fire.
The Russian assault may have been a rogue operation and it's not clear who was paying the Russian contingent, whether it was Russia directly, Syria, Iran or a third party.
At the time of the 7 February attack, the Russian defence ministry said "there are no Russian servicemen in Deir az-Zour."
However, many Russian citizens are fighting in Syria as mercenaries working for a private military company called Wagner, according to numerous reports.
The US is in talks with Russia now in search of an explanation for what happened, said two administration officials who asked not to be identified discussing private conversations.