US Syria strikes 'killed up to 200 Russian fighters' in Deir az-Zour

US military forces reportedly killed scores of Russians fighting on behalf of the Syrian regime last week in deadly clashes in the oil-rich region of Deir az-Zour, according to one US official and Russian sources.

More than 200 contract soldiers, mostly Russians, died in a failed attack on a base in the Deir az-Zour region held by US and Kurdish forces, two Russian sources told Bloomberg News.The incident, if confirmed, would be the deadliest clash between US and Russian civilians since the end of the Cold War.

The US official put the death toll in the fighting at about 100, with 200 to 300 injured, but was unable to say how many were Russian.

Russia's military said it had nothing to do with the attack on the base and the US accepted the claim. US Defence Secretary Jim Mattis called the whole thing "perplexing," but provided no further details.

"This is a big scandal and a reason for an acute international crisis," Vladimir Frolov, a former Russian diplomat and lawmaker who's now an independent political analyst, told Bloomberg News.

"But Russia will pretend nothing happened."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.