Russia's defense minister has met with Syrian President Bashar al-Assad during a visit to the Syrian capital.
Syrian state television said the previously unannounced visit by Sergei Shoigu on June 18 was to discuss military cooperation and efforts to fight "terrorism."
Meanwhile, the Pentagon has voiced "strong concerns" over Moscow's bombing of U.S.-backed forces in southern Syria.
Pentagon spokesman Peter Cook said on June 18 that U.S. military officials holding a video conference with Russia expressed the concern over attack on coalition-supported forces at the Al-Tanaf garrison.
U.S. defense officials said that Russian forces in Al-Tanaf targeted a meeting of combatants on June 16 that was held to coordinate the fight against forces of the Islamic State extremist group in Syria and Iraq.
The Syrians belonged to the New Syrian Army, trained by the British and the Americans in a coalition camp in Jordan, while the Iraqis were tribal fighters, U.S. officials said.
Russia's Defense Ministry said on June 16 that it had not carried out any strikes targeting opposition forces included in the cease-fire, without mentioning Al-Tanaf -- an assertion challenged by the United States.