"We... must now acknowledge that it is obvious that the destruction was not completely carried out," she said, noting that there is "strong evidence" that Damascus deployed the chemical weapons.

Damascus narrowly avoided US and French air strikes in 2013 in retaliation for a suspected sarin attack by agreeing to hand over its chemical arsenal.

Following Saturday's alleged chemical attack in rebel-held Douma, US President Donald Trump this week ratcheted up the pressure on Damascus, warning in a series of tweets that "missiles will be coming" in retaliation.

Merkel said a "full spectrum of measures" will be considered in response to the gas attack, but ruled out any military deployment.

"Germany will not participate militarily, but we will provide our support if the representatives in the UN Security Council ... were to take steps beyond diplomatic measures," she stressed.

Germany has deployed reconnaissance and refuelling flights as part of the international anti-jihadist operation in Syria.

But any further military role would require parliamentary approval.