PARIS (REUTERS) - France said on Tuesday it was 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, adding that the test results had been handed to the United Nations. "France is certain that sarin gas was used several times in Syria in limited areas."

United Nations investigators said on Tuesday they had "reasonable grounds" to believe that limited amounts of chemical weapons had been used in Syria in a conflict where brutality was now a tactic of war.

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, without specifying whether Paris was able to deduce who had used the gas.