Mattis, who made the comments as he testified before a House panel, appeared on Capitol Hill for the first time since a chemical attack in Syria left more than 40 dead and fueled talk of a USA -led strike against Syrian President Bashar Assad's regime.

"I believe there was a chemical attack and we're looking for the evidence", he said.

"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 USA forces deployed in the Middle Eastern country.

"We will not know from this investigating team" who was responsible, Mattis said, although he theorized that chlorine or sarin nerve agent - or a combination of the two - were used in the barrel bombing of Douma.

There have been several similar attacks this year, he told lawmakers, but the USA did not have troops in those locations because the U.S.is not engaged on the ground there.

In fact, Defense Secretary James Mattis told lawmakers today that he will provide the White House with military options against the Syrian regime, but the president has not yet settled on a course of action.

According to a report in Politico, Mattis has been pushing President Donald Trump and his advisors to look beyond the "spray and pray" approach of cruise missile attacks to consider what the U.S.is trying to achieve in Syria and whether it has a plan to achieve those aims.

Donald Trump said at a Cabinet meeting that "major decisions on Syria" will come in the next 24 to 48 hours after he figures out who was responsible for the alleged attack in Douma - Russia, or Syria, or Iran, or "all of them together".

At stake in Syria is the potential for confrontation, if not outright conflict, between the US and Russian Federation, former Cold War foes whose relations have deteriorated in recent years over Moscow's intervention in Ukraine, its interference in the 2016 USA presidential election and, most recently, its support for Syrian President Bashar Assad.

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While he believes the Syrian regime launched a chemical weapons attack on April 7, Mattis said inspectors with the Organization for the Prohibition of Chemical Weapons have yet to arrive on the scene.

Several lawmakers grilled the Pentagon chief on the legal authorities the military has for action in Syria beyond the remit of its current mission, which is to work to destroy the Daesh.

"Russia vows to shoot down any and all missiles fired at Syria", Mr. Trump wrote.

"Get ready Russian Federation, because they will be coming, nice and new and smart!" he tweeted, referring to US missiles. At a photo-taking session during a Pentagon meeting with his Dutch counterpart, Mattis was asked by a reporter whether he had seen enough evidence to blame the Syrian government. "Our strategy is to engage by, with and through allies in all things we do". "We're still working on this".

President Trump, on the other hand, has firmly blamed Assad and his ally Russian President Vladimir Putin for the alleged attack, and said the administration would decide on a response soon.

"Whenever the Syrian army achieves victory in the field, some western countries rise their voices and intensify their movements in an attempt to change the track of events", Assad said, according to SANA.

Asked to comment on possible US strikes, Peskov said "it's necessary to avoid any steps that may fuel tensions in Syria".

Mr. Trump did not detail what a strike on Syria would look like, or whether these would be US missiles.

Shortly after his tweeted warning to Russia, Mr. Trump took a more conciliatory tone in lamenting that the U.S. -Russia relationship "is worse now than it has ever been".