Syrian state media and a military media unit run by Lebanon's Hezbollah group said early on Thursday that the U.S.-led coalition targeted Syrian army positions in Syria's desert, but U.S. military officials denied any knowledge of the strikes.
The Britain-based Syrian Observatory for Human Rights says at least 12 pro-government fighters were killed in the airstrike. According to the monitoring group, none of the fatalities were Syrian nationals but foreign fighters.
A Syrian source close to the government, meanwhile, said the bombardment struck bases manned by the Iranian Revolutionary Guards allied with the Syrian government.
"Some of our military sites between Albu Kamal and Hamimia were exposed at dawn today to aggression launched by U.S. coalition jets," state news agency SANA reported, citing a military source.
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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 kilometers (60 miles) west of the Euphrates river where the coalition is backing ground forces against Islamic State.
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."
Syrian state media did not immediately report the strikes.
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 U.S. 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 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 towards it.