Russia said on Saturday it had told the United States it was unacceptable for Washington to strike pro-government forces in Syria after the US military carried out an air strike on pro-Assad militia last month.
Russian Foreign Minister Sergei Lavrov relayed the message to US Secretary of State Rex Tillerson in a phone call on Saturday initiated by the U.S. side, the Russian Foreign Ministry said in a statement.
US officials told Reuters last month that the US military carried out the air strike against militia supported by the government of President Bashar al-Assad which it said posed a threat to US forces and US-backed Syrian fighters in the country's south.
Russia said at the time that the US action would hamper efforts to find a political solution to the conflict and had violated the sovereignty of Syria, one of Russia's closest Middle East allies.
"Lavrov expressed his categorical disagreement with the US strikes on pro-government forces and called on him to take concrete measures to prevent similar incidents in future," the ministry said.