Syrian state media reported that the U.S.-led coalition fighting Islamic State struck Syrian army positions in eastern Syria early on Thursday, but the U.S. military denied knowledge of the incident.
The Britain-based Syrian Observatory for Human Rights said at least 12 pro-government fighters were killed in the airstrikes and that none of the fatalities were Syrian nationals but foreign fighters.
Islamic State lost most of its territory in Syria last year, but retained some remote desert areas and has attacked the Syrian army and allied forces in recent weeks. The coalition also recently restarted its own campaign against the jihadi group in Syria.
"Some of our military sites between Albu Kamal and Humeima were exposed at dawn today to aggression launched by U.S. coalition jets," state news agency SANA reported, citing a military source.
The strikes caused only material damage and came within 24 hours of an Islamic State attack on Syrian army positions in the same region, SANA reported.
A military media unit run by Lebanon's Hezbollah, an ally of Damascus, said the strikes were near T2, an energy installation near the border with Iraq about 100 kilometers (60 miles) west of the Euphrates.
A U.S. military official denied any knowledge of the strikes.
"We have no operational reporting of a U.S.-led coalition strike against pro-Syrian regime targets or forces," Captain Bill Urban, a spokesman for U.S. 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 mostly held by Islamic State until last year, when two rival military campaigns swept it from most of its territory, leaving only remnants in remote pockets of the desert.
The campaign by the Syrian army, backed by Russia, Iran and Shiite militias including Hezbollah, operated mostly on the west side of the Euphrates River.
A rival campaign by the Syrian Democratic Forces alliance of Kurdish and Arab militias, backed by the U.S.-led coalition, mostly took territory on the east side of the river.
The Syrian Observatory for Human Rights reported on Thursday that Islamic State combatants had been fighting pro-Syrian government forces to the west of the Euphrates, and the SDF to its east, on Wednesday night.
The U.S. 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 toward it.
