President Barack Obama has said the US has seen evidence of chemical weapons being used in Syria.
However, speaking after meeting Turkish PM Recep Tayyip Erdogan, he insisted it was important to get more specific details about alleged chemical attacks.
Earlier, residents of a north Syrian town told a BBC reporter how government forces had dropped poisonous gas canisters on them from helicopters.
The government has repeatedly denied claims it has used chemical agents.
It is unclear why the well-armed regime would deploy chemical weapons, which are illegal under international law.
But allegations of chemical attacks have been made against the government after assaults in various parts of Syria in March and May.
The US had warned that such a development would be a "red line" for possible intervention.
And Mr Obama reiterated on Thursday that a whole range of diplomatic and military options were available.
"Our militaries are constantly sharing information. We have seen evidence of the use of chemical weapons inside Syria," he said.
"Those chemical weapons inside of Syria also threaten our security over the long term as well as [that of] our allies and friends and neighbours."
However, he added that "more specific information" was needed.
Mr Erdogan had earlier claimed he had evidence of chemical attacks, and said he and Mr Obama would continue to talk about Syria later in the day.
Vomiting?
Earlier, the BBC's Ian Pannell was told by residents of Saraqeb town near Aleppo that regime had used poisonous gas in an attack on 29 April.
Doctors at the local hospital said they had admitted eight people suffering from breathing problems.
Some were vomiting and others had constricted pupils. One woman died.
Tests are being carried out in France, the UK and Turkey on samples from the site of the attack.
Residents have videos they allege show the gas attack.
Some of the videos show objects falling from helicopters, and others feature activists holding canisters.
The videos are impossible to verify.
Chemical weapons expert Hamish de Bretton-Gordon said the evidence from Saraqeb was strong, but incomplete.
He said the victims of four recent alleged attacks, including in Saraqeb, had shown symptoms consistent with "a nerve type of agent, be it sarin or be it organophosphate".
In March, Syria's government and opposition blamed each other for an alleged chemical attack in Khan al-Assal in the north, which killed at least 27 people.
Both sides called for an inquiry into the incident, and the UN assembled a 15-strong investigation team.
However, the Syrian government has refused the team access, apparently unhappy with their request for the right to look into all credible allegations.
The UN estimates that two years of conflict in Syria has left at least 80,000 people dead.
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