Russia said on Saturday it had told the United States it was unacceptable for Washington to strike pro-regime forces in Syria after the U.S. military carried out an air strike on pro-Assad militia last month.

Axar.az reports citing Yeni Safak.

Russian Foreign Minister Sergei Lavrov relayed the message to U.S. 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.

U.S. officials told Reuters last month that the U.S. military carried out the air strike against militia supported by the regime of President Bashar al-Assad which it said posed a threat to U.S. forces and U.S.-backed Syrian fighters in the country's south.

Russia said at the time that the U.S. 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 U.S. strikes on pro-[regime] forces and called on him to take concrete measures to prevent similar incidents in future," the ministry said.

The two men had also exchanged assessments of the situation in Syria, it added, and confirmed their desire to step up co-operation to try to end the conflict there.

The ministry said Lavrov and Tillerson had also discussed the need to try to mend the rift between Qatar and other Arab nations through negotiations, and had talked about the state of U.S.-Russia relations and planned meetings between officials from the two countries.