US Defense Secretary Jim Mattis has dismissed concerns the US is being dragged into a broader conflict in Syria after a major clash with pro-Syrian government forces overnight that may have left 100 or more of them dead.
The US-led coalition said it repelled an unprovoked attack near the Euphrates River by hundreds of troops aligned with Syrian President Bashar al-Assad, who were backed by artillery, tanks, multiple-launch rocket systems and mortars.
The incident underscored the potential for further conflict in Syria's oil-rich east, where the US-backed Syrian Democratic Forces (SDF) alliance of Kurdish and Arab militias holds swathes of land after its offensive against Islamic State.
The pro-government forces were "likely seeking to seize oilfields in Khusham" east of the Euphrates in Deir al-Zor province, a US official said.
Mattis described the attack on the US-backed fighters, who were accompanied by US special operations forces, as "perplexing" but he said the retaliatory US-led coalition strikes were defensive and limited in nature.
Asked whether the US military was stumbling into Syria's broader conflict, Mattis said: "No. This is self-defence.
"If we were getting involved in a broader conflict then it would have had an initiative on our part."
No US or US-backed forces died but the US official estimated more than 100 pro-Syrian government forces were killed in the counter-attack.
Syrian state television reported the coalition had caused "dozens of dead and wounded" by bombing pro-government forces, although a commander in the military alliance supporting Assad said seven members of pro-government forces were killed and 27 wounded.
In a letter to the United Nations, Syria's foreign ministry said the strike was a "war crime" and called for the coalition to be dismantled.
The US-led coalition was set up in 2014 to battle Islamic State fighters in Syria and Iraq, who were largely defeated last year.
About 2000 US forces remain on the ground in Syria, allied to the Kurdish-led SDF alliance.
The Syrian civil war, now entering its eighth year, has killed hundreds of thousands of people and driven more than 11 million from their homes, while drawing in regional countries and global powers supporting client factions on the ground.