Both Washington and Paris said Saturday that more countries were getting behind the need for military action after European Union nations condemned "the cynical use of chemical weapons".
US Secretary of State John Kerry said the number of countries ready to take military action was now in the "double digits", after holding talks in Lithuania with EU foreign ministers.
Following the meeting, the EU ministers issued a 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.
There were "a number of countries, in the double digits, who are prepared to take military action," he said.
"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, 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," Obama said. The president has just returned from a G20 summit in Saint Petersburg that deadlocked over 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.
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".