In response to the Syrian government's attack on United States-backed rebels, a USA -led coalition targeted pro-regime forces in airstrikes on Wednesday.
The US-led coalition killed at least 100 regime and allied fighters in retaliation for an attack in eastern Syria, a US official said, in one of the deadliest incidents of its kind.
SANA says the local fighters were battling IS militants and the US -backed Kurdish-led Syrian Democratic Forces in the area.
A USA military official who spoke on condition of anonymity said the United States bombing killed about 100 troops, but Veale said the military did not have an exact number for the casualties. They spoke on condition of anonymity as details are still emerging on the attack.
Syria's official state-run news agency SANA confirmed the deaths Thursday and labeled the strike an "aggression" and "new massacre". When the advancing forces unleashed a volley of 20-30 tank and artillery rounds toward the base, U.S. warplanes attacked, he said.
Syrian state television reported that the coalition had caused "dozens of dead and wounded" by bombing pro-government forces.
The Defense Ministry charged that the American aim in Syria was "capture and retention of economic assets".
Syrian state media characterized the event differently, saying the USA had bombed "popular local forces fighting" ISIS, and that it was a U.S. "attempt to support terrorism". Moscow and Washington maintain contacts in eastern Syria to prevent unexpected confrontation between forces they support.
She stressed that the coalition strikes were purely defensive and that Washington is not seeking to fight Syrian government forces. That official also said the military assesses that some of the Russian contractors were killed in the United States counterstrike.
Turkish officials say the leaders of Turkey, Russia and Iran will meet in Istanbul to discuss peace efforts for Syria. Mattis said the USA was still trying Thursday to establish exactly who the attackers were but added that the Russians have said they did not have forces there and "you can't expect somebody to deconflict something they can't control".
To complicate matters, two United States defense officials told CNN that the USA military now assesses that Russian contractors, believed to be funded by the Russian government, participated in Wednesday's assault.
Russia's Defence Ministry said the pro-government militias involved in the incident had been carrying out reconnaissance and their activities had not been previously agreed with Russian Federation.
"The recent incident once again shows that the United States' illegal military presence in Syria is actually aimed at taking control of the country's economic assets and not at fighting against the ISIS worldwide terror group", said a statement issued by the Russian Defense Ministry. Nouri Mahmoud, spokesman for the SDF's most powerful element, the Kurdish YPG militia, described the clash as "skirmishes" and said each side had returned to their former positions.
Russia's Interfax cited the Defence Ministry as saying the incident showed the USA goal in Syria was not to battle Islamic State but "the capture and withholding of the economic assets", an apparent reference to the Khusham oil field.