The French government said on Tuesday that its tests had confirmed the use of chemical weapons “on several occasions” in the Syrian conflict and that President Bashar al-Assad’s regime was responsible in at least one case.

Tests conducted on samples gathered by France had shown that “sarin gas has been used in Syria on several occasions in a localized manner,” Foreign Minister Laurent Fabius said in a statement.

The statement did not identify who carried out the attack or where it took place.

But in a later interview with France 2 television Mr. Fabius said that in at least one case “there is no doubt that it is the regime and its accomplices” who carried out the attack.

The United States reacted cautiously to the French announcement.

White House spokesman Jay Carney said more work needed to be done to establish who had used the chemical weapons, in what quantities they were used and under what circumstances.

“We need to expand the evidence we have, we have to make it reviewable, we have to have it corroborated,” he said. “We will continue in that effort.” Earlier, the UN Commission of Inquiry on Syria said there were “reasonable grounds to believe that limited quantities of toxic chemicals were used.” But it also said further investigations by a UN weapons panel were needed.

U.S. President Barack Obama had previously said that the use of chemical weapons would be a “red line” in the conflict, which could trigger a military intervention. Britain and France had echoed those warnings.

For Mr. Fabius, “a line has been crossed, without question.” In terms of a response, “all options are on the table,” he said.

They ranged from not reacting at all to a response “that includes the use of arms against the place where the gas is produced, where it is held, where it is stored,” he said.

Asked if that meant bombing regime facilities, he said: “We’re not there yet.” He also ruled out an immediate decision to arm the opposition.

“We must both react and at the same time not block the possible (Geneva) peace conference,” he said, adding France was talking with its partners, including the US and Britain, about the situation.

Both sides in the conflict have been accused of deploying chemical weapons, but most of the allegations have concerned al-Assad’s forces.

Le Monde newspaper last week reported that it had witnessed several toxic gas attacks against rebel forces in the Jobar district of Damascus in recent weeks.

The paper collected samples from doctors that treated victims, which were among those used in the government tests.

Mr. Fabius said he had handed the results Tuesday to Ake Sellstrom, head of the UN fact-finding mission into possible chemical weapons use in Syria.

“It would be unacceptable that those responsible for these crimes go unpunished,” he said.

The UN Commission of Inquiry, in its report, did not identify the kind of materials it suspected had been used in attacks but did list four suspected incidents of their use.

Two attacks allegedly took place in Aleppo and Damascus provinces on March 19, the third in Aleppo city on April 13, and the fourth in Idlib province on April 29, the commission said.

Commission member Carla Del Ponte, a former UN war crimes prosecutor, said in May that there were testimonies pointing to rebels using banned chemicals. But the commission said Tuesday there was “no compelling evidence” for this.

Sarin is a highly toxic agent, which affects the nervous system, overstimulation muscles and vital organs. In high doses, it can cause suffocation.

While Western powers had threatened possible intervention in the case of chemical weapons use they have balked at the growing prominence of an al-Qaeda affiliates on the rebel side.

The UN investigators, in their report to the UN Human Rights Council on Tuesday, warned the conflict had reached new levels of brutality.

“The desperation of the parties to the conflict has resulted in new levels of cruelty and brutality, bolstered by an increase in the availability of weapons,” the experts said.

Government forces and allied militias had committed war crimes and crimes against humanity, the commission said, pointing to massacres, the besieging of towns and the use of cluster bombs and other imprecise munitions.

Rebel groups have also committed various war crimes, especially by abducting alleged government collaborators, the report added.

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