John Kerry has said the number of countries prepared to take military action against Syria is now "in the double digits".
The US Secretary of State spoke after a day spent trying to bolster support from EU ministers after a G20 summit ended with no agreement on how to tackle the alleged use of chemical weapons.
Speaking in Paris, Mr Kerry said: "There are 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."
Mr Kerry held talks with 28 European foreign ministers in the Lithuanian capital Vilnius before travelling to Paris and London on Sunday to meet Arab leaders, including Palestinian President Mahmoud Abbas.
After the meeting, French foreign minister Laurent Fabius said there was agreement that all indications point to the Assad regime being responsible for a gas attack in Damascus that may have killed more than 1,400 people.
EU High Representative Catherine Ashton called for "a clear and strong response" to the attack but officials said the European Union has agreed that any potential strike against Syria should wait until after UN inspectors publish their report.
Respected German newspaper Frankfurter Allgemeine Sonntagszeitung has reported that the UN's chemical weapons experts could submit their initial findings by the end of next week.
Mr Kerry was boosted by Germany's decision after the meeting to back a strongly-worded statement issued at the end of the G20 summit in Russia, blaming the Syrian government.
He said: "We are very grateful for the statement that came out of the meeting today with respect to Syria, a strong statement about the need for accountability."
Later, Mr Kerry held further talks with Mr Fabius in Paris and spoke in French and English as he outlined the case for military action.
He said the world should not be "spectators to slaughter", but said President Obama had not yet decided whether to wait for the UN inspection report before taking action.
Middle East peace negotiations and other issues are on the agenda for Mr Kerry's talks at all three stops, but the ongoing crisis in Syria will overshadow his visit.
Meanwhile, President Barack Obama has arrived back in Washington after huge divisions emerged over the Syrian crisis at the G20 summit in St Petersburg, Russia.
In his weekly address to the American people, Mr Obama sought to emphasize that military action in Syria would not turn into a long-term conflict like Iraq or Afghanistan.
He said: "There would be no American boots on the ground. Any action we take would be limited, both in time and scope – designed to deter the Syrian government from gassing its own people again and degrade its ability to do so.
"We cannot turn a blind eye to images like the ones we’ve seen out of Syria. Failing to respond to this outrageous attack would increase the risk that chemical weapons could be used again, that they would fall into the hands of terrorists who might use them against us, and it would send a horrible signal to other nations that there would be no consequences for their use of these weapons."
A statement signed by 11 of the G20 nations said they hold Syrian President Bashar al Assad responsible for the alleged chemical attack.
The joint statement issued by Australia, Canada, France, Italy, Japan, South Korea, Saudi Arabia, Spain, Turkey, the UK and the US condemned the "horrific" attack in Damascus and called for a "strong international response", but it did not specify military action.
During the summit Mr Obama said the world could not "stand idly by", but Russian President Vladimir Putin warned it would be "outside the law" to attack without the UN's blessing.
Mr Putin also said Russia would "help Syria" if the US were to strike, pointing to existing military, economic and humanitarian co-operation.
Washington has prepared the ground for possible airstrikes, evacuating non-essential embassy staff from Beirut and urging Americans to avoid all travel to Lebanon and southern Turkey.
As well as the stubborn international differences, the US administration still has to win backing from Congress for any action against Syria.
Congress reconvenes on Monday and Mr Obama is to address the nation on Tuesday. But he acknowledged that convincing Congress to back military action against the Syrian regime would be a "heavy lift".
"I understand the scepticism," Mr Obama said.