Lt. Gen. Jeffrey Harrigian says the drone saw the tank "that took a shot at us", and that he believes the USA rules of engagement allowed for the tank to be destroyed in self-defense to protect the drone.
Mr Mattis said: "I don't have any reporting that some non-Russian Federation soldiers, but Russian contractors were among the casualties". I can't give you anything on that. "We have not received that word at Central Command or the Pentagon". In December previous year, President Vladimir Putin ordered the partial retreat of Russian troops from Syria. Circumstances of the conflict have not been established so far.
At least 100 Russian mercenaries were killed by American-backed military forces during an attack on a US-held base in Syria, according to reports.
The Pentagon earlier in the day declined to identify the 100 soldiers it said were killed by U.S. air strikes.
The Russian Defense Ministry has insisted that its troops weren't involved in the incident, saying 25 Syrian volunteers were wounded in the USA strike.
Mattis would not confirm whether Russian Federation did, or did not control the group of mercenaries.
The attack took place in the early hours of Thursday when 300-500 pro-Assad troops advanced with tanks and artillery on the Syrian Democratic Force (SDF) base.
It has stressed repeatedly since February 7 that no members of the Russian armed forces were killed and that any Russians fighting alongside the Syrians were mercenaries.
He said: "Those steps look increasingly like part of efforts to create a quasi-state on a large part of Syrian territory - from the eastern bank of the Euphrates River all the way to the border with Iraq".
In the Baltic Sea city of Kaliningrad, a local Cossack conservative group also reported losing one of its own, Vladimir Login, in the attack.
"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 Russian Federation and the United States is growing", Yavlinsky said in a statement.
In the past two weeks, in addition to the Iranian drone incident and the Israeli strikes, Kurdish fighters in Syria shot down a Turkish helicopter that was attacking them last week, killing two Turkish soldiers on board, and Syrian rebels shot down a Russian fighter jet. Those claims couldn't be verified.
But the Kremlin along with Russian President Vladimir Putin refuse to admit anything about the deaths. Most of the soldiers were said to be Russians who were fighting on behalf of Syrian leader Bashar al-Assad.
Harrigan added that the attacking forces were fighting on the side of Bashar al-Assad's regime. The firm has not commented on the reports. Strelkov wrote on his Vkontakte page on February 10 about "several hundred" killed and injured Russian mercenaries.
Ionov said not all those killed were Russian: Some of the paid fighters came from other countries that were once part of the Soviet Union.