UN investigators say they have "reasonable grounds" to believe that limited amounts of chemical weapons have been used in Syria.
But the UN Commission of Inquiry said in a report released on June 4 in Geneva that more evidence is needed to determine the precise chemical agents used and who used them.
The UN report said "war crimes and crimes against humanity have become a daily reality in Syria."
"Government forces, as you see in the report, and affiliated militia have committed murder, torture, rape, forcible displacement, enforced disappearances, and other inhumane acts," Paulo Pinheiro, the head of the commission, said on June 3 in Geneva ahead of the release of the report.
"Many of these crimes were perpetrated as part of widespread or systematic attacks against civilian populations and constitute crimes against humanity. War crimes and gross violations of international human rights law, including summary execution, arbitrary arrests and detention, unlawful attack, attacking protected objects, pillaging and destruction of property, have also been committed."
Pinheiro added that the rebels have also committed atrocities.
"Antigovernment armed groups have also committed war crimes, including murder, sentencing and execution without a due process, torture, hostage-taking, and pillage," he said. "They continue to endanger the civilian population by positioning military [targets] in civilian areas."
The commission, consisting of more than 20 investigators, examined four possible chemical attacks that reportedly took place in March and April.
The report, based on interviews with victims, medical staff, and other witnesses, covered the period from mid-January to mid-May.
French Foreign Minister Laurent Fabius later said France was "certain that sarin gas was used several times in Syria in limited areas."
Fabius said tests conducted in France on various samples from Syria confirmed the presence of sarin gas.
He did not provide details about where or when the sarin was used but said it had happened "several times in a localized manner."
Following Fabius's comments, the United States expressed caution in ascribing blame to the gas attacks.
"The French report that you're citing says that more work needs to be done to establish who is responsible for the use [of sarin], the amount that was used, and more details about the circumstances around it," White House spokesman Jay Carney said at a news briefing.
Carney also said Washington was urging the Syrian authorities to lift the siege of the town of Qusair near the border with Lebanon.
Syrian government forces launched an offensive to chase rebels from Qusair nearly three weeks ago and Syrian authorities are now claiming its troops are close to recapturing the town after fierce fighting.
The United Nations estimates that more than 80,000 people have been killed in Syria's two-year civil war.
With reporting by Reuters and AP