Russia on Thursday said that the U.S. has been training the militants at its At Tanf military base in the southeastern Syria.
Russian Foreign Ministry spokeswoman Maria Zakharova said Moscow is concerned about the U.S. unwillingness to decide the problems, existing around "the occupied territory".
"A large military base is deployed in the sovereign territory of a UN member state, in a strategically important area at the tri-border of Syria, Iraq and Jordan, near the route, connecting Baghdad and Damascus, which, according to numerous evidence, is preparing militants," she said.
According the datum published earlier by Anadolu Agency, some 7,700 square kilometers of the At Tanf region in the Syrian-Iraqi border are currently under U.S.-backed opposition control.
In early September, the U.S.-led anti-Daesh coalition began war games at At Tanf garrison in what it said are exercises aimed at honing "warfighting skills for counterterrorism operations".
Despite the stated "counterterrorism" aim of the exercises, CNN reported they are actually a direct response to Russian military threats.
Moscow reportedly sent a written note to the U.S. warning that it would carry out "precision strikes" on militants around the At Tanf area despite a roughly 55-kilometer (34-mile) de-confliction zone around the base.