US Defense Secretary Jim Mattis told lawmakers in Congress that the US military does not have evidence that either chlorine or sarin were used in an alleged chemical attack in the Syrian town of Douma, adding that the Pentagon's only proof that such an incident occurred has come from social media and media reports.

"There have been a number of these attacks. In many cases, you know we don't have troops, we're not engaged on the ground there, so I cannot tell you that we had evidence, even though we had a lot of media and social media indicators that either chlorine or sarin were used," Mattis said, speaking to members of the House Armed Services Committee on Thursday.

The defense secretary said that he did believe that a chemical attack did take place, but that the US was still "looking for the actual evidence."

"We're still assessing the intelligence, ourselves and our allies. We're still working on this," he reiterated.

Warning that he was concerned that a US strike might lead to an "out of control" escalation in the Syrian war, Mattis said that Washington was "committed to ending that war through the Geneva process through the UN orchestrated effort."

"On a strategic level, [the issue is] how do we keep this from escalating out of control, if you get my drift on that," he said, likely alluding to the prospects of a confrontation between Russian and US forces deployed in the Middle Eastern country.

Mattis said he would discuss Syria options at a National Security Council meeting later Thursday, and promised to keep Congressional leaders informed if the Pentagon did decide to strike.

Asked if the US was ready for an attack, Mattis replied that "We stand ready to provide military options if they're appropriate, as the president determined."

Emphasizing that the use of chemical weapons was "simply inexcusable," the defense secretary also accused Moscow of complicity in Syria's alleged retention of a chemical weapons stockpile.

Syrian opposition media reports last week of an attack by government forces involving chemical weapons prompted the US and its allies to blame Damascus and begin preparations for a possible military response.

The Syrian government denied responsibility.

The Russian Center for Reconciliation sent inspectors to Douma, finding no trace of chemical weapons use.

Moscow has called for an independent investigation into the matter.

Syria destroyed its chemical weapons stockpile in 2013 in a deal brokered by Russia and the United States in exchange for the latter's agreement not to attack the Middle Eastern country.

In 2014, the Organization for the Prohibition of Chemical Weapons confirmed that Syria's chemical weaposn arsenal had been eliminated.