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

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

The report by the United Nations' independent commission of inquiry on Syria said the panel had not been able to ascertain which chemicals had been 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 between March and April this year. Other reported instances are still being investigated, and it is of the "utmost import" that Syria grant access to the U.N. team of experts tasked with probing the allegations, it said.

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

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

"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."

Obama warned last year that 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. 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 deliberate targeting of civilian neighborhoods have become "a daily reality," with both sides responsible for abuses, the report said, although it attributed blame for most of the violence to 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 the rebels set to expire in August, a "diplomatic surge" is urgently needed to bring both parties to the negotiating table, it said.

On Tuesday, however, Russian President Vladimir Putin indicated that Russia may not proceed with the controversial delivery of the S-300s, saying that 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 has also expressed concerns that the missile system could be transferred to Lebanon's Hezbollah, enabling the Shiite militant group to strike deep inside Israeli territory. Assad hinted in an interview last week that the missiles had already arrived.

Russia is one of Assad's chief backers and has also 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 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 this year.

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