"We consider the fact that, despite the preliminary arrangements, the United States did invite [the Revolutionary Commando Army], which is translated as Partisans of Revolution, for the protection of convoys inside the so-called exclusive zone completely unacceptable," Zakharova said.
She added that Moscow also had questions regarding Washington's refusal to let the representatives of the Syrian Arab Red Crescent charity in the Rukban refugee camp located near al-Tanf.
"This means that they have something to hide," she indicated.
The US military currently occupies a 34-mile zone around its base in al-Tanf, where it has been training Syrian opposition forces since 2016.
The base has become the subject of criticism as Damascus and Moscow have expressed concerns that Washington was "spewing Daesh* mobile groups who make inroads to launch subversive terrorist operations against Syrian troops and civilians."
*Daesh (also known as ISIS/ISIL/IS) is a terrorist group banned in Russia