"Three employees received non-life threatening injuries. Right now an investigation of the incident is under way," according to the statement, reported by Al Jazeera.
State Department spokeswoman Margie Harf condemned the attack "against individuals or facilities protected by international law."
"The United States continues to emphasize that those responsible for atrocities on all sides must be held accountable," she said in a statement Sunday.
Moscow has been one of Syria's strongest allies since the beginning of the civil war between anti-government rebels and forces loyal to Syrian President Bashar Assad.