Syrian rebels prepare to fire locally made rockets, in Idlib province, northern Syria on June 4, 2013. (Edlib News Network/via AP)

The French government said Tuesday that it has confirmed the use of sarin gas by Syria’s government, and a U.N. panel reported that it has “reasonable grounds” to believe that chemical weapons have been used in the country’s civil war, deepening international suspicions that the red line set by President Obama has been breached.

France “now is certain that sarin gas was used in Syria multiple times and in a localized way,” according to a statement issued by French Foreign Minister Laurent Fabius, which said tests carried out by a French laboratory on samples taken from victims showed the presence of the nerve gas.

The report by the United Nations’ independent commission of inquiry on Syria said the panel could not ascertain which chemicals were used, how they were delivered or by whom because it did not have access to samples taken directly from victims.

But it said it had “reasonable grounds to believe that limited quantities of toxic chemicals were used” on at least four occasions from March to April. Other reports of chemical weapons use are still being investigated, the U.N. commission said, adding that it is of the “utmost import” that Syria grant access to the U.N. team ofexperts tasked with probing the allegations.

Fabius said his government is confident that the regime of Syrian President Bashar al-Assad was responsible for at least one of the two instances in which France had confirmed the use of the gas.

Interactive Grid: Keeping track of the conflict in Syria through videos, images and tweets.

“In the second case, there is no doubt that it is the regime and its accomplices,” Fabius told the France 2 television station in an interview. “We have integrally traced the chain, from the attack, to the moment people were killed, to when the samples were taken and analyzed.”

His comments followed an eyewitness account by two reporters with France’s Le Monde newspaper describing how canisters containing small quantities of what appeared to be a nerve agent had repeatedly been fired at rebel positions during their two-week stay with opposition fighters in the eastern suburbs of Damascus, a hotly contested battleground where the regime has made significant advances in recent weeks.

The reporters said they had taken samples from the victims for analysis in France, and Fabius said the samples were one of two sources for France’s conclusion that sarin has been used.

“All options are on the table,” he said when asked what France’s response might be. “That means we could decide not to intervene, or we could decide to intervene, including militarily, where the gas is produced or stored.”

Later Tuesday, Britain’s U.N. Ambassador Mark Lyall-Grant said his government also has acquired evidence that small amounts of sarin have been used in Syria.

“The evidence that we have suggests that there is a use of a number of different variants of chemical agents, a combination of agents, . . . sometimes including sarin, sometimes not,” he told reporters at U.N. headquarters.

Obama warned last year that the use of chemical weapons represented a “red line” that would trigger U.S. intervention, but U.S. officials have repeatedly stressed that they do not have convincing evidence that banned chemicals have been used in Syria. The latest reports are likely to compound pressure on the administration to do more to influence the outcome of the conflict, which the U.N. report said continues to escalate to “new levels of brutality.”

The report painted a bleak picture of an increasingly complex and cruel war in which sectarian sentiments are deepening, atrocities are becoming more common and the threat of a regionwide conflict is growing.

War crimes, including murder, torture, rape and the targeting of civilian neighborhoods, have become “a daily reality,” with both sides responsible for abuses, the report said, although it blamed most of the violence on Assad’s government.

The report noted evidence that the rebels have carried out summary executions, hostage-taking and at least one beheading in which a 14-year-old decapitated two government soldiers. But, it said, “the violations and abuses committed by anti-government armed groups did not reach the intensity and scale of those committed by Government forces and affiliated militias.”

It cited the government’s “indiscriminate” use of a wide range of heavy weapons, including cluster munitions, surface-to-surface missiles and thermobaric bombs, most of them targeting civilian neighborhoods, as one of the panel’s chief concerns.

With Russia threatening to ship potentially game-changing S-300 antiaircraft missiles to the Syrian government and a European Union embargo on arms sales to rebel groups lapsing, the report said, a “diplomatic surge” is urgently needed to bring both parties to the negotiating table.

On Tuesday, however, Russian President Vladimir Putin indicated that Moscow may not proceed with the controversial delivery of the S-300s, saying it has not yet “implemented” its long-standing contract to do so.

“It is a very serious weapon,” he said after a summit with European leaders in the Russian city of Yekaterinburg. “We do not want to upset the balance of power in the region.”

Israel also has expressed concerns that the missile system could be transferred to Lebanon’s Hezbollah, enabling the Shiite militant group to strike deep inside Israeli territory.

Russia is one of Assad’s chief backers and has supplied his regime with much of the ammunition and conventional weaponry it has used to launch recent offensives against the rebels and reverse many of the gains the opposition achieved over the past year.

In a separate report released Tuesday, New York-based Human Rights Watch accused the Syrian government of responsibility for the deaths of at least 147 people whose bodies were fished out of a river in the city of Aleppo between January and March.

Most of the victims had their hands tied behind their backs, gunshot wounds to their heads and tape across their mouths, the report said, adding that a Human Rights Watch investigation suggested they had been thrown into the river in government-controlled areas of the divided city.

Will Englund in Moscow, Colum Lynch at the United Nations and Ahmed Ramadan in Beirut contributed to this report.