American troops may have killed hundreds of Russian mercenaries during air strikes in Syria, a US army general has said.
Up to 300 pro-regime fighters are thought to have died in American bombing that was launched in response to a surprise attack on a US-held base in the oil-rich Deir Ezzor region last month.
The US launched artillery fire and air strikes at the Russian force after coming under attack on 7 February.
"Those [Russian] artillery rounds could have landed and killed Americans and that's why we continue to prepare our defences," Brigadier General Jonathan Braga told NBC News.
Many of those killed are believed to have been Russian mercenaries employed by a contracting firm. Moscow insisted its regular soldiers were not involved.
However, BGen Braga said he was “absolutely concerned” the conflict could have grown into a larger confrontation with Russia.
The clash is believed to have been the deadliest between US and Russian forces since the end of the cold war.
The Kremlin has only confirmed the deaths of “several dozen” of its citizens, but continued to insist its military was not involved. It initially said it had no information on the deaths of any Russian mercenaries.
The Pentagon, meanwhile, said the air strikes were retaliation for an “unprovoked” advance on land controlled by the US-backed Syrian Democratic Forces, a coalition of Kurds and Syrian rebels.
Neither Moscow nor Washington have publicly offered many details about the clash.
“We only deal with the data that concerns Russian forces servicemen,” said President Vladimir Putin’s spokesman Dmitry Peskov. “We don’t have data about other Russians who could be in Syria.”
It later said five of its citizens had been killed, a figure it subsequently increased to “several dozen”.
BGen Braga however, said that reports that between 200 and 300 of the Russian forces had been killed was "close to our estimates.”
Not all of those killed were mercenaries working for a Russian firm, but Bloomberg reported that more than 200 of those who died were contract fighters.
The US deployed around 2,000 troops to Syria to fight Isis, which has now been forced out of much of the territory it controlled.
While the US opposes Bashar al-Assad's Syrian government, American forces have not been targeting his regime's soldiers or those of its civil war allies, such as Russia.
