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US Defence Secretary Jim Mattis said Friday that sarin gas might have been used in a recent attack in Syria, and warned that Washington will not tolerate the use of deadly chemicals by Bashar al-Assad's forces.

He also claimed that chlorine had been weaponised and repeatedly used by the Syrian regime in recent weeks. 

"We are even more concerned about the possibility of sarin use, and we are looking for the evidence," he added.

The Pentagon is looking into reports that sarin has been used by the Syrian regime in recent attacks after claims were put forward by Syrian opposition activists and NGOs.

"We are not refuting them - we are looking for evidence of it since clearly we are dealing with the Assad regime that has used denial and deceit to hide it," he said.

"They would be ill-advised to go back to violating the chemical convention."

Mattis' warning comes after another US official said Washington might take military action against the Syrian regime following recent chlorine and suspected sarin attacks.

President Donald Trump "hasn't excluded anything" to halt chemical attacks, a senior US official told AFP.

"Using military force is something that is still considered."

US officials also believe the Syrian regime could have developed and used new chemicals.

Senior US sources told the agency that rather than barrel bombs, the regime could be using mortars and missiles to deliver the chemicals in strikes on opposition areas.

Investigators have reported 260 chemical attacks in Syria, the vast majority attributed to the Assad regime.

The widespread use of chemical warfare by the Syrian regime threatens international taboos on their use, Pentagon officials fear.

US cruise missiles were launched on a Syrian airbase last year after a sarin chemical attack on the opposition village of Khan Sheikhoun in Idlib province.

Up to 100 civilians were killed in last April's sarin attack.

Hundreds more died in other chemical weapon strikes on eastern Ghouta in 2013, with the US came within a whisker to responding with military action.

More recently, chlorine gas has been released on the besieged opposition suburb injuring dozens.