MOSCOW (AP) — An unknown number of private Russian military contractors were killed by a U.S. strike in Syria, Russian media reported Tuesday in a development that could further inflame Russia-U.S. tensions.

Officials in both Moscow and Washington remained coy about the deaths, which would be an embarrassment for both countries if it turns out that Russian fighters were part of a unit that attacked a military force with American troops.

President Vladimir Putin's spokesman, Dmitry Peskov, refrained from commenting on reports about the incident last week, saying they need to be verified.

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