The United States and France have warned of a strong response after an alleged deadly chemical attack by the regime on the rebel-held town of Douma on Saturday.

"The buildings of the ministry of defence and the army headquarters have been empty for two days," the Syrian Observatory for Human Rights monitoring group said.

Regime forces have also evacuated military airports as well as the bases of the elite Fourth Division and Republican Guard outside the capital, the Observatory said.

There was no immediate confirmation from the Damascus regime.

A source with pro-regime forces told AFP: "The Syrian army has taken protective measures especially for military airports and bases."

The source said they expected the Americans would inform the Russians before any strike.

US President Donald Trump said "missiles will be coming" in response to the alleged chemical attack, which rescuers say killed more than 40 people.

Last year, Trump launched a cruise missile strike against a Syrian air base in retaliation for a sarin attack the United Nations later pinned on Assad.

More than 350,000 people have been killed in Syria's conflict since it started in 2011 with the brutal repression of anti-government protests.

It has since spiralled into a complex war involving world powers.