Russia conceded Tuesday that “dozens” of its citizens were injured and an unspecified number were killed in a U.S. strike in Syria earlier this month, though it insisted its military was not involved in the clash.

“In the course of the recent military clash, in which the servicemen of the Russian Federation did not participate in any way and the technical means were not used, there are dead citizens of Russia and the [Russian Commonwealth] countries, as already mentioned,” the Russian Foreign Ministry said in a statement Tuesday. “There are also several dozens wounded.”

Russia previously downplayed its citizens' role in the incident, saying only that five may have died. But reports citing friends of the deceased and injured as well as unnamed officials have said “scores” of Russian mercenaries were killed or injured in the attack.

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U.S. officials, meanwhile, have refused to identify the makeup of the force that attacked U.S.-backed forces in Syria, which prompted the U.S. military strike, nor have American officials ascribed motive or said who was directing the attackers.

Speaking to reporters traveling with him back to Washington on Saturday after a trip to Europe, Defense Secretary James Mattis James Norman MattisBiden courts veterans amid fallout from Trump military controversies Trump says he wanted to take out Syria's Assad but Mattis opposed it Gary Cohn: 'I haven't made up my mind' on vote for president in November MORE said he didn't know who directed the attack, but cast doubt on the idea that the mercenaries were working on their own.

“I still cannot give you any more information on why they would do this,” Mattis said over the weekend. “But they took direction from someone. Was it local direction? Was it from external sources? Don't ask me. I don't know."

“But I doubt that 257 people all just decided on their individual own selves to suddenly cross the river into enemy territory and start shelling a location and maneuvering tanks against it.”

At issue is a Feb. 7 incident in Deir ez-Zour province, where pro-Syrian government forces and U.S.-backed forces have been converging to eliminate Syria’s last remaining pockets of ISIS fighters. The United States and Russia have agreed to “deconflict” the area by having each of its forces stay on separate sides of the Euphrates River.

But on Feb. 7, the pro-government forces crossed the river and launched an “unprovoked” attack on the Syrian Democratic Forces, which is backed the United States. The U.S. military responded by striking the pro-government forces.

Asked Saturday whether Russia has responsibility for contractors in Syria, Mattis said he needs more information to “understand and answer that authoritatively.”