US Secretary of State John Kerry says he and Arab League foreign ministers have agreed that the Syrian president's alleged use of chemical weapons crossed a "global red line".
Mr Kerry, speaking in Paris, is in Europe to muster support for action against President Bashar al-Assad.
"Assad's deplorable use of chemical weapons crosses an international, global red line," he said.
Mr Assad has reportedly again denied any link to the attack.
In an interview for US broadcaster PBS, to be broadcast on Monday, Mr Assad also reportedly "suggested that there would be, among people that are aligned with him, some kind of retaliation if a strike was made", PBS said.
Arab countries are divided on the question of military strikes on Syria.
The BBC's Hugh Schofield reports from Paris that some like Saudi Arabia and Qatar are in favour while others like Syria's neighbours Jordan and Lebanon are far more cautious, worried about the conflict spreading across their borders.
The US accuses Mr Assad's forces of killing 1,429 people in a sarin gas attack on 21 August.
Mr Assad's government blames the attack on rebels fighting to overthrow him in the country's two-and-a-half-year civil war, which has claimed some 100,000 lives, according to UN estimates.
There are reports that rebel forces have taken control of the historic Christian town of Maaloula, north of Damascus.
'No military solution'
Mr Kerry was speaking at a news conference after meeting Arab League foreign ministers.
Referring to Mr Assad's regional allies in Lebanon and Iran, Mr Kerry said: "It is clear that if we don't take action, the message to Hezbollah, Iran, Assad will be that nobody cares that you have broken this 100-year-old standard."
Syria's civil war, he said, was going to require a political solution.
"We have repeated and I repeat every time I stand up and talk about it - there is no military solution," Mr Kerry continued.
"What we are seeking is to enforce the standard with respect to the use of chemical weapons."
He said that all the Arab League foreign ministers at the meeting had agreed without exception that the use of chemical weapons had crossed "an international global red line".
Iranian Foreign Minister Mohammad Javad Zarif has said on a visit to Baghdad that a US strike on Syria would be contrary to the UN charter.
In Maaloula, about 55km (34 miles) north of Damascus, Islamist militants from the Nusra Front and other rebels took control after several days of fighting, the UK-based opposition Syrian Observatory for Human Rights said.
Syrian government troops had pulled back to the outskirts of the village, Rami Abdurrahman told the Associated Press.
'Talks on track'
The Paris meeting was arranged some time ago primarily to discuss the Israeli-Palestinian peace process.
On that issue, Mr Kerry said Israel and the Palestinians are "steadfast and determined" in their commitment to talks and a two-state solution.
He said he intended to meet Israeli Prime Minister Benjamin Netanyahu "shortly".
Israeli and Palestinian negotiators restarted peace talks on 14 August in Jerusalem under US mediation, after a gap of nearly three years.
As part of the process, Israel agreed to release dozens of long-serving Palestinian prisoners while the Palestinians were expected to refrain from upgrading their membership of UN agencies.
However, disagreement over the construction of new settler homes in the West Bank and East Jerusalem has dogged the negotiations.
In a joint statement with Mr Kerry, the Arab League foreign ministers expressed their "full support" for his efforts but expressed concern about continued Israeli settlement activity and "unilateral Israeli actions in Jerusalem that create a negative environment".
About 500,000 Jews live in more than 100 settlements built in the West Bank and East Jerusalem. The settlements are considered illegal under international law, though Israel disputes this.