During a visit to Germany on Monday, U.S. President Barack Obama confirmed that 250 more personnel would be sent to Syria for the fight against the Islamic State (IS) militant group.
Reports said that there are already 150 U.S. soldiers deployed in Kurdish areas in northern Syria.
"We are concerned over the fact that the U.S. carries out such actions without coordination with the legitimate government of the Syrian Arab Republic, which is a violation of sovereignty of Syria," Ryabkov said.
Nevertheless, he stressed that Moscow and Washington still maintain military cooperation in Syria on a daily basis, according to the Tass news agency.
According to news reports, the U.S. personnel are obliged to supervise the implementation of a safe-zone in the northern part of Syria and oversee the nationwide ceasefire among the opposing groups as well.
In response to the U.S. decision on troop deployment in Syria, the Syrian government on Thursday condemned the entry of 150 U.S. soldiers into Kurdish areas in northern Syria as "flagrant aggression" and "outright assault violating the Syrian territories."