Hundreds of Russian mercenaries were reportedly killed by U.S. forces in a failed attack on a U.S.-held military base and refinery last week. Citing U.S. and Russian sources, Bloomberg News reported more than 200 contract killers—mostly Russian citizens fighting for Syrian President Bashar al-Assad—died in what is believed to be the deadliest incident between Russian and U.S. citizens since the Cold War. Russia’s military has disowned any responsibility for the attack in the Deir al-Zour region and a U.S. military spokesperson appeared to accept that claim in a statement. No U.S. coalition casualties have been reported from the attack, but the death toll from the mercenary side is still rising, with soldiers being treated in hospitals in Moscow and St. Petersburg. Defense Secretary James Mattis is reported to have called the incident “perplexing.” The reported death toll would be far greater than all of the Russian casualties the Kremlin has acknowledged during its Syrian incursion.