Syrian state media and a military media unit run by Lebanon's Hezbollah group said early on Thursday that the US-led coalition targeted Syrian army positions in Syria's desert, but US military officials denied any knowledge of the strikes.
"Some of our military sites between Albu Kamal and Hamimia were exposed at dawn today to aggression launched by US coalition jets," state news agency SANA reported, citing a military source.
The media unit run by Hezbollah, a military ally of Damascus, said the strikes were near T2, an energy installation located near the border with Iraq and about 100 km (60 miles) west of the Euphrates river where the coalition is backing ground forces against Islamic State.
The Syrian Observatory for Human Rights said at least 12 pro-regime fighters were killed in the strike.
A US military official denied any knowledge of the strikes.
"We have no operational reporting of a US-led coalition strike against pro-Syrian regime targets or forces," Captain Bill Urban, a spokesman for US Central Command, told Reuters.
A Pentagon spokesman, Marine Maj. Adrian Rankine-Galloway, said: "We have no information to substantiate those reports."
A war monitor, the Syrian Observatory for Human Rights, reported two waves of US-led missile attacks early Thursday it said targeted areas controlled by ISIS east of the Euphrates River.
Syrian state media did not immediately report the strikes.
If confirmed, the attacks would be the first by the US on Syrian army positions since April, when the US, Britain and France bombed government facilities in retaliation for a suspected gas attack blamed on Syria's government.
Eastern Syria was mostly held by Islamic State until last year, when two rival campaigns, one by the Syrian army backed by Russia, Iran and Hezbollah, the other by Kurdish and Arab militias backed by the US coalition, took most of its land.
Communication between Russia and the United States averted most clashes between them. However, the coalition has struck Syrian pro-government forces that it said were attempting to attack coalition positions.
The US military operating outside the coalition also maintains a base at Tanf in the eastern Syrian desert near the borders with Iraq and Jordan and last year struck pro-government forces moving along a road towards it.