WASHINGTON -- U.S. air strikes killed multiple Russia mercenary soldiers serving in Syria, according to news reports and open-source researchers, with some reports saying dozens may have died.
President Vladimir Putin's spokesman denied knowledge of the February 7 incident in the Deir al-Zor region, whose mystery has deepened in recent days, as relatives and colleagues of Russians soldiers serving in Syria have begun speaking out.
The U.S. Defense Department said the air strikes, which included fighter and ground-attack aircraft, and Marine artillery, targeted Syrian government-backed troops after as many as 500 attackers launched a coordinated assault on a base housing Syrian opposition forces, along with U.S. military advisers in Deir al-Zor.
U.S. officials said around 100 soldiers were killed, but did not identify them.
"I will not speculate on the composition of this force or whose control they were under," Lieutenant General Jeffrey Harrigian, a spokesman for U.S. Central Command, said on February 13. "We are focused on a singular enemy: ISIS. We're not looking for a fight with anyone else, but as [U.S. Defense] Secretary [Jim] Mattis said last week: 'If you threaten us, it will be your longest, and your worst, day.'"
Over the weekend, however, reports emerged on Russian blogs and elsewhere that an unknown number of Russian contract soldiers were killed.
The Russian Defense Ministry has said that none of its soldiers was in the area and suggested that that pro-Syrian government fighters may have failed to coordinate their actions with the Russian military.
U.S. military officials also said that, during the air assault, they had been in contact with the Russian military, via a special hotline set up by the two sides to avoid hostile incidents between the two.
The Moscow-based Conflict Intelligence Team, which monitors Russian involvement in the conflicts in Syria and Ukraine, on February 12 identified three Russians it said were among those killed.
A Cossack organization in Kaliningrad posted on social media that one of its members had also been killed.
President Vladimir Putin has said repeatedly that the Russian intervention in Syria, which began in late 2015, was winding down, and in December ordered that uniformed Russian troops and equipment be withdrawn.
Meanwhile, an unknown number of Russians working for private military companies have been serving alongside regular Russian troops. A St. Petersburg company known as PWC Vagner has been one of the most prominent.
The Russian newspaper Novaya Gazeta said 13 Russians were killed in the attack and another 15 wounded.
Bloomberg, citing two unnamed Russians, said more than 200 contract soldiers died in the attack. The New York Times cited a Syrian military officer as saying about 100 Russian soldiers had been killed.
Grigory Yavlinsky, a liberal Russian politicians who is running in next month’s presidential election, also suggested there were Russian deaths in Syria. He called on Putin to report publicly about "the actions of Russian troops in Syria at present and the number of deaths of Russian citizens regardless of their military status."
"I also think it is essential to account publicly on interactions with the United States, since the danger of an accidental or intentional direct military engagement between Russia and the United States is growing," Yavlinsky said in a statement.
Asked about the reports, Kremlin spokesman Dmitry Peskov told journalists in Moscow on February 13 that the government only knows about the actions of the Russian military in Syria.
"We don't have information about other Russians who might be in Syria," he said.
The reports of Russian deaths in the incident in Deir al-Zor came amid persistent tension in U.S.-Russian relations, which are strained by disputes over Syria and several other issues.
They also come weeks ahead of a March 18 election in which Putin is widely expected to secure a fourth term as president.
With reporting by Bloomberg, AP, Interfax, and CBS News