France said it had proof President Bashar al-Assad's regime has used the deadly nerve agent sarin gas in Syria's civil war, adding that "all options," including armed intervention, are on the table.
Washington said it needed more evidence before concluding that sarin has been used.
Earlier, a UN report said there were "reasonable grounds" to believe both sides in Syria had used chemical weapons.
But French Foreign Minister Laurent Fabius told France 2 television: "We have no doubt that the gas is being used. The conclusion of the laboratory is clear: there is sarin gas."
Experts had analysed samples brought back from Syria, he said.
Earlier, his ministry released a statement from Fabius in which he said "France is now certain that sarin gas has been used in Syria several times and in a localised fashion".
Although the statement did not say who had used the gas, it added: "It would be unacceptable that those guilty of these crimes could benefit from impunity."
Later, in his televised remarks, Fabius went further, saying that in at least one case, there was "no doubt that it was the regime and its accomplices".
A diplomatic source said this particular case was an April 29 helicopter attack by regime forces on Saraqeb, in the northwest province of Idlib.
"A line has been indisputably breached," said Fabius.
"We will hold talks with our partners on what we must do and all options are on the table," he said, to decide "whether to react, including in an armed manner".
"But we are not there yet," Fabius added, referring to armed intervention.
"We must react, but at the same time we must not block an eventual peace conference."
Officials are heading to Geneva for a preparatory meeting on the US- and Russia-backed peace conference on Syria.
Officials hope to hammer out terms to get President Bashar al-Assad's camp and the rebels to negotiate directly for the first time.
White House spokesman Jay Carney said Washington needed to know more before declaring that sarin gas had been used in Syria.
"We need to expand the evidence we have ... before we make any decision," Carney said, alluding to President Barack Obama's stated position that use of the deadly nerve agent would be a "game-changer" for Washington.
French diplomats said the samples tested by the French laboratory had come from two locations -- Jobar, a suburb of Damascus, and Saraqeb.
The samples from near Damascus were brought back to France by two journalists with the French daily Le Monde. They reported the use of chemical weapons in the area in mid-April.
There were no details on how France obtained the other sample.
Fabius said he had sent their results to the United Nations, which is also investigating the issue.
UN investigators said earlier Tuesday they had "reasonable grounds" to believe that both sides in Syria had used chemical weapons, the first time they have made such an allegation since beginning their work in 2011.
It called on Syria to let UN investigators enter Syria to check for the possible use of chemical weapons. So far, the regime has denied them access.
A spokesman for UN Secretary General Ban Ki-moon said he had read the UN document and "he finds the catalogue of atrocities in that report to be both sickening and staggering".
British Foreign Secretary William Hague also called on Syria to admit UN investigators.
He also urged Syria to let aid workers into the city of Qusayr, where regime forces and rebel fighters were continuing their battle, echoing a call Monday from US Secretary of State John Kerry.
Of the UN report, he said: "The accounts of war crimes and crimes against humanity, and the use of chemical weapons, rape, murder and executions are utterly appalling and underline the urgent need for a political solution to end the conflict."
Russian President Vladimir Putin said Tuesday that Moscow had so far refrained from supplying the powerful S-300 surface-to-air missile systems to Damascus.
Speaking after talks with European Union leaders in the city of Yekaterinburg, Putin said Moscow had signed the missile contract several years ago, but it had not yet "been realised".
"We do not want to upset the balance in the region," he said.
His comments appear to back off from Moscow's earlier position that the missiles would be a "stabilising factor" that would deter foreign intervention in Syria.
Russia's veto at the UN Security Council has shielded Assad's regime from repeated attempts to impose UN sanctions.
Syria's civil war has already cost more than 94,000 lives in 26 months of fighting.
The Syrian Observatory for Human Rights said shellfire near the Russian embassy in Damascus had killed a civilian and wounded a member of the security forces.
A representative of the Russian embassy in Damascus told AFP two Syrian security guards had received injuries but that no embassy staff had been killed or hurt in the attack.
Turkey's army said Tuesday it had returned fire after shots were fired from over the border in Syria at its military vehicles. No one was hurt in Monday's incident, it added.