The Pentagon voiced "strong concerns" over Moscow's bombing of US-backed forces in southern Syria, a spokesman said on June 18.
U.S. military officials holding a video conference with Russia "expressed strong concerns about the attack on the coalition-supported counter-ISIL forces at the Al-Tanf garrison, which included forces that are participants in the cessation of hostilities in Syria," said Pentagon spokesman Peter Cook after the talks.
He added that the Pentagon "emphasized that those concerns would be addressed through ongoing diplomatic discussions on the cessation of hostilities."
US defense officials said that Russian forces in Al-Tanf targeted a meeting of combatants supported by the US-led coalition that was held to coordinate the fight against forces of the Islamic State 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, officials said.
Russia's defense ministry said late Thursday however that it had not carried out any strikes targeting opposition forces included in the ceasefire, without mentioning Al-Tanf -- an assertion challenged by the United States.
"Regarding safety, department officials conveyed that Russia's continued strikes at Al-Tanf, even after US attempts to inform Russian forces through proper channels of ongoing coalition air support to the counter-ISIL forces, created safety concerns for U.S. and coalition forces," Cook said.
"(Defense) Department officials requested Russian responses to address those concerns. The two sides reiterated the need to adhere to measures to enhance operational safety and avoid accidents and misunderstandings in the air space over Syria."