There is no doubt Syria's government has used sarin during the country's crisis, says France's foreign minister.
Laurent Fabius said lab tests in Paris confirmed numerous uses of the nerve agent, adding that those who resort to chemical weapons must be punished.
But he did not specify where or when the agent had been deployed; the White House has said more proof was needed.
The UK also says it has tested samples which give evidence of the use of sarin in Syria.
According to a Foreign Office spokesman, Britain "has obtained physiological samples from inside Syria which have tested positive for the nerve agent sarin".
The UK statement added: "There is a growing body of limited but persuasive information showing that the regime used - and continues to use - chemical weapons" in Syria.
In a new report, the UN commission of inquiry on Syria also urged foreign powers not to increase the availability of arms in Syria.
UN Secretary General Ban Ki-Moon described the atrocities listed in the report - which details evidence of fresh suspected massacres, sieges and violations of children's rights - as "sickening and staggering", said his spokesman.
Children have been taken hostage, forced to watch torture and even participate in beheadings, says the report.
Others have been killed while fighting in the two-year uprising against President Bashar Assad's regime that the UN says has left at least 80,000 people dead.
Sarin-use referred to UN
Mr Fabius said various samples taken from unspecified locations in Syria and tested in France showed the presence of sarin.
"There is no doubt that it's the regime and its accomplices" that were responsible, he told France 2 television in an interview.
Mr Fabius said the test results had been handed to the UN.
"All options are on the table," he added. "That means either we decide not to react or we decide to react including by armed actions targeting the place where the gas is stored."
The tests came after journalists for French newspaper Le Monde smuggled blood and urine samples out of Syria following what they believed were chemical attacks in the capital Damascus and the northern town of Saraqeb.
Sarin, an extremely potent chemical nerve agent, is colourless and odourless. The use of chemical weapons is banned by most countries.
Both the Syrian government and the rebels have in the past accused each other of using the weapons.
The US was continuing its effort to gather evidence about the likely use of chemical weapons, the White House said on Tuesday.
The US and the UK have said there is emerging evidence of Syrian government forces having used sarin, with Washington saying it had "varying degrees of confidence" that chemical weapons had been deployed.
President Barack Obama called in April for a "vigorous investigation", saying the use of such weapons would be a "game changer" if verified.
Mr Assad's government says the claims do not have any credibility, denouncing them as "lies".
A UN team established to look specifically into the issue of chemical weapons had previously said it was ready to go to Syria, but wants unconditional access with the right to inquire into all credible allegations.
Russia and the US are leading an international push for a peace conference on Syria, possibly to be held in Geneva in the next few weeks.
Lakhdar Brahimi, the UN's special envoy for Syria, is holding closed-door talks with US and Russian diplomats in Geneva later on how to get Syria's warring parties to the negotiating table.
That will not be easy, says the BBC's Imogen Foulkes in Switzerland: the opposition groups in Syria still do not agree on who should represent them.
There are also tensions between the EU and Russia over the EU's plan to lift an arms embargo on the opposition, and between Russia and the US over Moscow's commitment to supply air defence systems to the Syrian government.
The UN human rights investigators, who have documented war crimes committed by both sides in Syria, say sending weapons to anyone involved in the fighting would be completely wrong.
Meanwhile, fighting continued around the strategically important western town of Qusair.
Lebanon's al-Manar TV, which is owned by Iranian-backed Shia militants Hezbollah, said the group's fighters and government troops had taken control of south-western parts of the town.