German Chancellor Angela Merkel on Sunday criticised the timing of European Union members' out-of-sync statements calling for a "strong" response to an alleged chemical weapons attack by the Syrian regime.

With the United States seeking to rally international support for punitive strikes on Syria in the wake of what it says was a deadly chemical attack by the regime on August 21, four EU countries signed a statement Friday at a G20 summit calling for a "strong" response.

The Washington-backed statement was signed by a total of 11 countries that day at the summit in Saint Petersburg, Russia -- but not by Germany, which is a G20 member but waited until Saturday to sign.

Merkel said she wanted to get all 28 EU members on board before signing -- which happened Saturday when EU foreign ministers issued a similar statement at talks in Lithuania calling for "a clear and strong response."

"I don't believe it's right for five countries to agree on a united stance without the other 23 that can't be there, knowing that 24 hours later all 28 will be gathering around the same table," Merkel, who is seeking re-election in two weeks' time, told a campaign rally in western Germany.

"That's why I said, 'Let's see to it that we have a united stance by all 28.'"

The remarks were a veiled criticism of Britain, France, Italy and Spain, the four EU countries that signed the G20 statement on Friday.

Neither the G20 statement nor the EU statement -- issued after a meeting attended by US Secretary of State John Kerry -- called for military intervention.

The EU statement called the August 21 attack outside Damascus "a war crime and a crime against humanity" and said there was "strong evidence that the Syrian regime is responsible."

"The international community cannot remain idle," it added.

"A clear and strong response is critical to make clear that such crimes are unacceptable and that there can be no impunity."