Washington: US Defense Secretary Jim Mattis dismissed concerns on Thursday that the United States was 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.

Assad, who is supported by Russia and by Shi’ite militias backed by Iran, has said he wants to take back every inch of Syria.

The pro-government forces were ‘likely seeking to seize oilfields in Khusham’ east of the Euphrates in Deir al-Zor province, said a US official on condition of anonymity.

US Senator Tim Kaine, who sits on Senate foreign relations and military oversight committees, said the episode raised serious concerns about the open-ended US military presence in Syria.

‘I am gravely concerned that the Trump administration is purposefully stumbling into a broader conflict, without a vote of Congress or clear objectives,’ Kaine said.

Mattis described the attack on the US-backed fighters, who were accompanied by US special operations forces, as ‘perplexing.’ But he described the retaliatory US-led coalition strikes as defensive and limited in nature.

Asked whether the US military was stumbling into Syria’s broader conflict, Mattis said: ‘No. This is self-defense.’

‘If we were getting involved in a broader conflict, then it would have had an initiative on our part,’ Mattis told reporters at the Pentagon.

No US or US-backed forces died but the US official who spoke anonymously estimated that more than 100 pro-Syrian government forces were killed in the counter-attack.

Syrian state television reported that the coalition had caused ‘dozens of dead and wounded’ by bombing pro-government forces. But a commander in the military alliance supporting Assad disputed the death toll, saying seven members of the pro-government forces were killed and 27 injured.

In a letter to the United Nations, Syria’s foreign ministry described the strike as a ‘war crime’ and called for the coalition to be dismantled, Syrian state news agency SANA said. ‘We demand (that the international community) condemn this massacre and hold the coalition responsible for it.’