US Defense Secretary James Mattis said Thursday that, although he believes that the Syrian regime staged a chemical attack on the town of Douma last weekend, authorities are still working to gather evidence and that can only be done by a team of inspectors.

"I believe there was a chemical attack and we are looking for the actual evidence," Mattis said in an appearance before the House Armed Services Committee, at which he expressed his hope that Syrian President Bashar al-Assad will authorize an investigation into the matter by the Organization to Prohibit Chemical Weapons (OPCW).

"As each day goes by - as you know, it is a non-persistent gas - so it becomes more and more difficult to confirm it," Mattis told lawmakers.

According to the defense secretary, the OPCW is still trying to get the authorization of the Syrian government to launch an investigation and if that is forthcoming inspectors could begin the probe within a week.

When questioned whether the US is considering acting unilaterally against Assad's forces, Mattis replied that the Pentagon's aim is to act with its "allies," presumably Britain and France, but he refused to provide details about the possible involvement of London or Paris because of the "confidential" nature of White House discussions on the matter.

Mattis said that the Donald Trump administration's "priority," just as was the case for the Barack Obama administration, in Syria is to destroy the Islamic State and the US military will "not engage in the civil war itself" in that country.

However, he added that the US cruise missile strike on a Syrian airbase in response to the deadly sarin gas attack on civilians by the Assad regime in April 2017 was justified because some things - such as chemical weapons attacks - are "simply inexcusable beyond the pale" and "in the worst interest ... of civilization itself."