Moscow- US forces killed scores of Russian contract soldiers in Syria in what may be the deadliest clash between citizens of the former foes since the Cold War, one US official and three Russians familiar with the matter said.

More than 200 mercenaries, mostly Russians fighting on behalf of Bashar Al Assad, died in a failed attack last week on a base held by US and US-backed forces in the oil-rich Deir Al Zor region, two of the people said.

The Russian assault may have been a rogue operation, underscoring the complexity of a conflict that started as a domestic crackdown only to morph into a proxy war involving extremists, stateless Kurds and regional powers Iran, Turkey and now Israel. Russia’s military said it had nothing to do with the attack and the US military accepted that claim. Defence Secretary Jim Mattis called the whole thing “perplexing,” but provided no further details.

“Coalition officials were in regular communication with Russian counterparts before, during and after the thwarted, unprovoked attack,” US Colonel Thomas F. Veale, a military spokesman, said in a statement. “Russian officials assured coalition officials they would not engage coalition forces in the vicinity.”

The offensive began late February 7 when the enemy fired rounds and moved toward the base in a “battalion-sized dismounted formation supported by artillery, tanks, multiple-launch rocket systems and mortars,” Veale said. No fatalities were reported on the coalition side and “enemy vehicles and personnel who turned around and headed back west were not targeted.”

The death toll from the incident, already about five times more than the official losses suffered by Russia’s military since it entered the war in 2015, is still rising, according to one mercenary commander. He said by phone on condition of anonymity that dozens of his wounded men are still being treated at military hospitals in St. Petersburg and Moscow.

Many of the Russians killed or injured were veterans of the Ukraine conflict, according to Alexander Ionov, the head of a Kremlin-funded organization that fosters ties to separatists who’s fought alongside pro-Al Assad forces in Syria.