Air strikes took place in the southern city of Albu Kamal in Deir Ezzor province
An air strike on Syrian army positions in eastern Syria overnight killed at least 12 pro-government fighters, the Syrian Observatory for Human Rights said on Thursday.
The Britain-based monitoring group said the strike was carried out by the US-led coalition in an area south of Albu Kamal which has repeatedly been attacked by the Islamic State militant group.
The new strikes comes shortly after Lebanon's Hezbollah group claimed warplanes from the coalition had targeted two Syrian army positions in the eastern Syrian desert on Wednesday.
Hezbollah, a Damascus ally, said the strikes took place near T2, an energy installation located near the border with Iraq and about 100km (60 miles) west of the Euphrates River where the coalition is backing ground forces against IS.
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.
Another Pentagon spokesman, who spoke on condition of anonymity, said: "We have no information to substantiate those reports."
Eastern Syria was held mostly by IS 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 back most of its land.
Communication between Russia and the United States averted most clashes between them. Still, the coalition has on occasion 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.