PARIS » Samples taken from Syria and tested in France have confirmed that sarin gas has been used there multiple times, France’s foreign minister said today.

Laurent Fabius said the tests carried out by a French laboratory “prove the presence of sarin in the samples in our possession.” He said France “now is certain that sarin gas was used in Syria multiple times and in a localized way.”

The brief statement concluded: “It would be unacceptable that those guilty of these crimes benefit from impunity.”

Earlier today, a U.N. report on Syria said there are “reasonable grounds” to believe that limited quantities of toxic chemicals have been used as weapons in at least four attacks in Syria’s civil war, but that more evidence is needed to determine the precise chemical agents used or who used them.

The U.N. Commission of Inquiry said conclusive findings can be reached only after testing samples taken directly from victims or the site of the alleged attacks. It called on Damascus to allow a team of experts into the country, saying lack of access continues to hamper the commission’s ability to fulfill its mandate.

President Barack Obama had previously declared the use of chemical weapons his “red line” for forceful U.S. intervention in Syria. He has since come under pressure to take action amid mounting allegations that President Bashar Assad’s regime has crossed that line.

The U.N. report appeared to strengthen the Obama administration’s argument that that the existing evidence is insufficient.

The commission’s report to the Human Rights Council on violations in Syria’s conflict accused both sides of committing war crimes. In an apparent message to European countries considering arming Syrian rebels, the report warned that the transfer of arms would heighten the risk of violations, leading to more civilian deaths and injuries.

“War crimes and crimes against humanity have become a daily reality in Syria where the harrowing accounts of victims have seared themselves on our conscience,” the report said. “There is a human cost to the increased availability of weapons,” it added.

The commission said it relied for the report on first-hand accounts to corroborate incidents, and carried out 430 interviews in the region and from Geneva, including via Skype and over the telephone, with victims and witnesses inside the country.

It also collected photographs, video recordings, satellite imagery and medical records. Reports from governments and non-government sources, academic analyses, and U.N. reports, including from human rights bodies and humanitarian organizations, also formed part of the investigation.

U.N.-Secretary General Ban Ki-moon has appointed a U.N. team to investigate alleged chemical weapons attacks in Syria after the government in Damascus asked him to investigate a purported attack by rebels on March 19 on Khan al-Assal village near the northern city of Aleppo. But the Syrian government insists that a probe be limited to that incident.

Syrian soldiers were reportedly killed and injured in the incident, which the rebels blame on government forces. Opposition activists have claimed there have been more than six instances when regime forces used chemical weapons.

Ban is insisting on a broader investigation, which would also include a December incident in Homs. He appointed Swedish chemical weapons expert Ake Sellstrom to lead a U.N. investigation. Syria has refused to allow his team into the country.

Last week, Syrian Foreign Minister Walid al-Moallem said Iraq under Saddam Hussein inadvertently paved the way for the 2003 U.S.-led invasion by allowing U.N. inspectors into the country, and suggested Syria is not about to make the same mistake. “We will not allow teams of inspectors to come to Syria to do whatever they want,” he said in a TV interview.

Syria is widely believed to have one of the world’s largest arsenals of chemical weapons, including mustard and nerve gas. The Assad regime has denied using such weapons during the civil war.

The confirmed use of chemical weapons could escalate the international response to the more than two-year-long conflict, which has killed more than 70,000 people, according to the U.N. Obama has said their use would be a “red line,” but the administration says it still looking for solid evidence.

Explaining its position in an April letter to two U.S. senators, the administration referred to intelligence assessments concluding “with varying degrees of confidence” and based in part on physiological samples that the regime has used chemical weapons on a small scale, specifically the nerve agent sarin. The letter said such assessments are not sufficient grounds for action because it is not clear how the exposure occurred and under what circumstances.

Since then, the governments of Britain, France and Turkey have also said there are indications of chemical weapons use, but that more testing is required.

“There are reasonable grounds to believe that chemical agents have been used as weapons,” the report said. “It has not been possible, on the evidence available, to determine the precise chemical agents used, their delivery systems or the perpetrator.”

The report said there are allegations of government forces using chemical weapons in four instances, but also did not rule out rebels using them.

“It is possible that anti-government armed groups may access and use chemical weapons …. though there is no compelling evidence that these groups possess such weapons or their requisite delivery systems,” the report said.

“Conclusive findings — particularly in the absence of a large-scale attack — may be reached only after testing samples taken directly from victims or the site of the alleged attack,” it said.

The report, covering the period from mid-January to mid-May, accused both sides of committing war crimes. On the government side, the report accused government forces and affiliated militia of committing torture, rape, forcible displacement and enforced disappearance. On the rebel side, the report accused armed groups of carrying out sentencing and execution without due process, as well as committing torture, taking hostages and pillaging.

It said violations and abuses by the rebels “did not, however, reach the intensity and scale of those committed by government forces and affiliated militia.”

“A dangerous state of fragmentation and disintegration of authority prevails in areas under anti-government armed groups control, despite attempts to fill the vacuum left by the withdrawal of the state through creating local councils,” it said.

AP correspondent Zeina Karam reported from Beirut.