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.

"The Russians told the Syrian army's leadership they had intelligence on possible US and French targets in Syria," Observatory head Abdel Rahman said.

According to the Russian intelligence, the United States is pushing for possible strikes to target the ministry of defence and army headquarters in Damascus, he said.

The intelligence indicated they were also aiming to strike military airports and bases, the Observatory said.

According to the same Russian intelligence, France would like targets to include President Bashar al-Assad's chemical facilities.

These would be the Syrian Scientific Studies and Research Center in Jamaraya, just north of the capital, and chemical warfare headquarters in Damascus.

Paris would be pushing for a strike on the Dumair military airport, which is suspected of having been the launchpad for Saturday's alleged chemical attack.

There was no immediate confirmation from the Damascus regime.

Rescuers say more than 40 people were killed in Saturday's alleged toxic gas attack.

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.