Defense Secretary Jim Mattis has said that the United States is still investigating who directed an attack on a base housing U.S.-backed opposition forces and U.S. military advisers in Syria earlier this month.

"Whatever happened, we'll try to figure it out. We'll work with, obviously, anyone who can answer that question," Mattis said on February 17 while flying back to Washington after a weeklong tour of Europe.

He said that Russian officers whom the United States coordinated with were apparently unaware of the February 7 attack in the Deir al-Zor Province, according to a Pentagon transcript.

The Pentagon has said that air strikes launched after as many as 500 pro-government forces attacked the base killed about 100 of the attackers.

The Russian Foreign Ministry said last week that five Russians who were not military personnel may have been killed in the clash, and claimed that reports putting the Russian toll in the dozens or hundreds was "disinformation."

Associates of Russian mercenaries fighting alongside Syrian government forces have said that there were large-scale casualties among the contractors in the clash.

"I understand that the Russian government now is saying some of their not military forces, but contractors, were involved in that still unexplained attack," Mattis said.

The attackers "took direction from someone.Was it local direction? Was it from external sources?Don't ask me.I don't know," the U.S. defense secretary said.

"But I doubt that 257 people all just decided on their individual own selves to suddenly cross the [Euphrates] River into enemy territory and start shelling a location and maneuvering tanks against it," he added.

Russia has given President Bashar al-Assad's government crucial support throughout the 7-year-old war in Syria, which began with a government crackdown on protests.

Moscow helped turn the tide of the conflict in his favor by a launching a campaign of air strikes in 2015 and stepping up its military presence on the ground.

The United States has backed rebel groups -- including the Syrian Democratic Forces (SDF), an alliance of Syrian Arab and Kurdish fighters -- fighting to topple Assad.

An SDF base near the Euphrates River "deconfliction line" set to avoid clashes between the two sides in Deir al-Zor Province was the site of the February 7 incident.

With reporting by Reuters and AFP