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.