PARIS -- The United States and France said Saturday that international backing was growing for military strikes to punish the Syrian regime for an alleged chemical attack, after European Union nations called for a "strong" response. US Secretary of State John Kerry said the number of countries ready to take military action was now in the "double digits", after earlier attending a meeting where EU foreign ministers united to call for action against Syrian President Bashar al-Assad's regime.

The EU did not explicitly back military action, but Kerry said he was "encouraged" by the "very powerful statement" made by the bloc.

He said there was "a number of countries, in the double digits, who are prepared to take military action. We have more countries prepared to take military action than we actually could use in the kind of military action being contemplated."

The US accuses the Assad regime of gassing more than 1,400 people to death in an August 21 attack outside Damascus.

In his weekly address, US President Barack Obama warned of the dangers of turning "a blind eye" to chemical attacks.

"I call on members of Congress, from both parties, to come together and stand up for the kind of world we want to live in," the president said after returning from a G20 summit in Saint Petersburg that deadlocked over the response to the Syria crisis.

Obama has asked for Congress to authorise strikes on Syria. The legislature reconvenes Monday and the president is set to address the nation Tuesday about the US response.

Kerry said Obama had made no decision about whether to wait for the release of a UN investigation into the August attack before taking action.

"The president has given up no right of decision in respect to what he will do," Kerry said at a joint press conference with French counterpart Laurent Fabius.

French President Francois Hollande, who has said Paris will wait for the results of the UN probe, said he expected the report to be ready by next weekend.

Hollande said he also expected a US Congress vote on "Thursday and Friday".

-- Facing a 'Munich moment' --

Outlining his case in Paris in both French and English, Kerry compared the situation to the 1938 Munich Agreement, which ceded control of part of Czechoslovakia to Nazi Germany.

"This is our Munich moment, this is our chance to join together and pursue accountability over appeasement... This is not the time to be silent spectators to slaughter," said Kerry.

Fabius told reporters there was "wide and growing support" to take action on Syria.

"Right now, seven of the eight countries in the G8 share our opinion on a strong reaction and 12 countries of the G20 also share this opinion," he said.

Seeking to whip up support from Washington's divided allies in Europe, Kerry held talks with the EU's 28 foreign ministers in Lithuania and with Fabius in France before heading to Britain on Sunday.

Split between Paris and London, who are hardline supporters of US-led military action, and nations reluctant to act without a United Nations mandate, the EU ministers managed to hammer out a compromise in Lithuania.

A statement read out by EU foreign policy chief Catherine Ashton dubbed the suspected chemical attack "a war crime and a crime against humanity".

There was "strong evidence that the Syrian regime is responsible," the statement said.

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

In Riyadh, the Gulf Cooperation Council urged the international community to intervene immediately to "rescue" the Syrian people from "oppression".

Pope Francis meanwhile led Catholics worldwide in a day of fasting and prayer for peace in Syria joined by Jews, Muslims and Orthodox Christians.

"In beloved Syria, in the Middle East, in all the world, let us pray for reconciliation and peace, let us work for reconciliation and peace," the pope told thousands of people massed for a vigil in St Peter's square.

Faced with a war-weary US public and little international support, Obama is bracing for an uphill battle to convince American lawmakers to back military action against Assad's regime.

According to a Washington Post survey, 224 of the current 433 House members were either "no" or "leaning no" on military action as of Friday. A large number, 184, were undecided, with just 25 backing a strike.

In France, the sole EU nation determined to join a US-led strike, the latest public opinion poll showed 68 percent of people opposed to military action, an increase of nine percentage points since late August.

The EU statement said there could be no end to the Syrian war without a political solution and urged the UN Security Council "to fulfill its responsibilities", a reference to Russia and China's repeated refusals to sanction Assad. (AFP)