U.S. forces killed scores of Russian contract soldiers in Syria last week in what may be the deadliest clash between citizens of the former foes since the Cold War, according to a U.S. official and three Russians familiar with the matter.

More than 200 mercenaries, mostly Russians fighting on behalf of Syrian leader Bashar al-Assad, died in a failed attack on a base and refinery held by U.S. and U.S.-backed forces in the oil-rich Deir Ezzor region, two of the Russians said. The U.S. official put the death toll at about 100, with 200 to 300 injured.

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 Islamic 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 U.S. military accepted the claim. Defense 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,” U.S. 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 about 8 kilometers (5 miles) east of the Euphrates River de-confliction line late on Feb. 7, when adversaries fired rounds and advanced 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 government in Damascus called the U.S. action “barbaric aggression” and a “war crime.”

The death toll from the incident, already about five times more than Russia’s official losses 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-Assad forces in Syria. It’s not clear who was paying the soldiers of fortune, whether it was Russia directly, its allies in the war, Syria and Iran, or a third party.

Reports in local media have said that Wagner — a shadowy organization often referred to as Russia’s answer to Blackwater, the U.S. military company now called Academi — was hired by Assad or his allies to guard Syrian energy facilities in exchange for oil concessions.

There’s a refinery in Deir Ezzor that once funded Islamic State operations that’s now “crucial” to Assad’s plans to finance the reconstruction of Syria once a peace deal is finally reached, according to Yury Barmin, a Middle East analyst at the Russian International Affairs Council in Moscow.

‘Big Scandal’

Russia’s Defense Ministry seemed to refer to the refinery in its statement about the attack, accusing the U.S. of using its “illegal presence” in Syria as an excuse to “seize economic assets” instead of fighting terrorists.

Vladimir Frolov, a former Russian diplomat and lawmaker who’s now an independent political analyst in Moscow, said the clash marked the first such armed exchange between the two powers since the Vietnam War.

“This is a big scandal and a reason for an acute international crisis,” Frolov said. “But Russia will pretend nothing happened.”

President Vladimir Putin’s spokesman, Dmitry Peskov, declined to comment on reports that Russian nationals were killed in Syria, saying the Kremlin only tracks data on the country’s armed forces.

Source www.bloomberg.com