US Secretary of State John Kerry has said the number of states ready to take military action against Syria's government is in the "double digits".
Speaking in Paris, he said the world could not be "silent spectators to slaughter" after Syria's alleged use of chemical weapons against its civilians.
The United States accuses President Bashar al-Assad's forces of killing 1429 people in a gas attack on 21 August.
The BBC reports the EU foreign ministers say there should be no action before a UN report.
French President Francois Hollande, a key ally for the US on military action against Mr Assad, has said he expects the preliminary UN report into the incident to be submitted at the end of next week.
Mr Kerry believes the world is facing a "Munich moment" in the Syrian crisis - a reference to the policy of appeasement that failed to stop Nazi Germany in the 1930s.
"We in the United States know ... that this is not the time to be silent spectators to slaughter," Mr Kerry said in Paris on Saturday after meeting foreign minister Laurent Fabius.
"This is the time to pursue a targeted and limited but clear and effective response that holds dictators like Bashar Assad responsible for the atrocities which they commit."
A G20 summit in Russia failed to produce international agreement on military action, with US President Barack Obama at odds with Russia's President Vladimir Putin, who blames the gas attack on rebels.
Mr Obama has said any military action will 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".
According to the UN, about 100,000 people have been killed in the Syrian conflict in the past 2½ years.
