He also suggested that contractors or employees of Russian private military companies have Hassama was not.
Mattis called the situation "perplexing", noting that the reported attack on the SDF base, which allegedly brought about the deadly American retaliation, "makes no sense, it does not appear to be anything coordinated by the Russians". These claims can not be verified.
The Pentagon says a counterattack aimed at defending US forces in the oil-rich Deir el-Zour province a week ago went on for more than three hours and involved B-52 strategic bombers, AC-130 gunships, F-15E attack planes, Apache attack helicopters and Reaper drones.
"We don't have data about other Russians who could be in Syria".
More details about those killed in the strikes began to emerge in the following days.
A Cossack organization in Kaliningrad posted on social media that one of its members had also been killed, and the independent Russian newspaper Novaya Gazeta, citing unnamed sources, reported that 13 Russians were killed and another 15 wounded.
Both sides are fighting the last remnants of the Islamic State group in Syria.
A representative of the Syrian Foreign Ministry, Ayman Soussan, said during Tuesday's press conference "we are not bound by anything that is formed by foreign sides, whatever their name or state, we are not bound by it, and it is of no concern to us".
No U.S or allied troops were hurt in the incident, but three people inside the tank were killed by a U.S. MQ-9 Reaper drone, a defense official told Fox News on Tuesday.
A Syrian military officer said that about 100 Syrian soldiers had been killed in the fighting on February 7 and 8, but news about Russian casualties has dribbled out only slowly, through Russian news organizations and social media.
What are Russia's casualties in the Syrian conflict? "All the media reports, though, need to be verified".
But analysts believe Russian Federation is using so-called private security firms to avoid deaths among its conscript army which could prove unpopular with its citizens - especially in the year of its presidential election.
The Russian Defence Ministry has insisted its troops were not involved in the incident.
What happened in the USA strikes?
"Now we hear completely different explanations from our USA partners regarding their presence in Syria - they say that this presence should be maintained not only until military tasks are accomplished, but also until there is a stable political process that should end in a transition of power which will be acceptable for all - meaning for the United States - which is regime change".
"We are focused on a singular enemy - ISIS", said the Air Force general.
"Those steps look increasingly as part of efforst 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", he went on.
Russian Federation officially withdrew regular forces from Syria in December after two years of fighting in support of President Bashar al-Assad, but hundreds of private military contractors and mercenaries remained.
"I'm sure the number of those confirmed dead is going to increase", said Alexander Ionov, a Russian anti-globalization activist who has fought in Syria and is well-connected among former fighters from eastern Ukraine. I can't give you anything on that. "We have not received that word" at key USA military headquarters, including the Pentagon. "(...) You can't ask Russian Federation to prevent the conflict in the case when she is not in control, it could not be done", said Mattis.