Germany will not join any military strikes against the Syrian government in response to a poison gas attack on a rebel-held town but supports Western efforts to chemical weapons use is unacceptable, Chancellor Angela Merkel says.

"Germany will not take part in possible - there have not been any decisions yet, I want to stress that - military action," she said after meeting Danish Prime Minister Lars Lokke Rasmussen in Berlin on Thursday.

"But we support everything that is being done to show that the use of chemical weapons is not acceptable," she added.

Meanwhile, US President Donald Trump has not yet made his "final decision" on how to respond to the alleged chemical attack.

Trump was to confer with French President Emmanuel Macron and British Prime Minister Theresa May on the action the allies might take, White House press secretary Sarah Sanders said.

The White House announcement came after May held an emergency cabinet meeting on the situation in Syria.

AAP

Dozens of people were killed last weekend in a suspected toxic gas attack on Douma, the largest town in a former rebel stronghold outside Damascus.

Moscow has warned against any escalation that could trigger a conflict between the United States and Russia and has called for an emergency UN Security Council meeting on Friday.