France says it is certain that the nerve agent sarin has been used in Syria on several occasions following tests it has carried out on samples recovered from the country.

"These tests show the presence of sarin in various samples in our possession," Foreign Minister Laurent Fabius said in a statement on Tuesday, adding that the test results had been handed to the United Nations.

The Syrian regime and the opposition have traded accusations that the other side have used chemical weapons during the two-year-long conflict.

"We have no doubt that the gas is being used ... the laboratory tests are clear," he told France 2 television about the bllod and hair samples later on Tuesday. "There is no doubt that the regime and its accomplices" are using them, he added.

France has been testing samples of suspected chemical weapon elements for several weeks, including some smuggled out by reporters from the French daily Le Monde.

"It would be unacceptable that those guilty of these crimes remain unpunished," Fabius said.

Use of chemical weapons is illegal under international law.

An exiled chemist who worked on developing Syria's chemical weapons told Al Jazeera last month that the country's stockpile comprises 700 tonnes of sarin agent.

The French statement came as UN investigators said there was mounting evidence that both sides have committed massacres, engaged in torture and used chemical weapons.

The UN Commission of Inquiry on Syria said that military and government leaders must be held accountable for implementing a "concerted policy" of human rights violations.

"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," its report said.

Investigators said they had "reasonable grounds" to believe that limited amounts of chemical weapons had been used in Syria.

In their latest report based on interviews with victims, medical staff and other witnesses, they said they had received allegations that Syrian government forces and rebels had used the banned weapons, but that most testimony related to their use by President Bashar al-Assad's forces.

The inquiry said conclusive findings could 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.