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.

Earlier on Tuesday, United Nations human rights investigators said they had "reasonable grounds" to believe that limited amounts of chemical weapons had been used by government forces in Syria.

In their latest report, based on interviews with victims, medical staff and other witnesses, they said they had received allegations that both Syrian government forces and rebels had used the banned weapons, but that at least four instances related to their use by state forces.

"There are reasonable grounds to believe that limited quantities of toxic chemicals were used. It has not been possible, on the evidence available, to determine the precise chemical agents used, their delivery systems or the perpetrator," Paulo Pinheiro, who chairs the UN commission of inquiry, told a news conference in Geneva.

The commission examined four reported toxic attacks in March and April but could not determine which side was behind them.

"The witnesses that we have interviewed include victims, refugees who fled some areas, and medical staff," Pinheiro said, declining to be more specific for reasons of confidentiality.

The team, composed of more than 20 investigators, conducted 430 interviews from January 15 to May 15 among refugees in neighboring countries and by Skype with people still in Syria.

U.K. 'shocked' at chemical weapon use

Britain's UN ambassador has said his government is shocked by the growing evidence of the repeated use of chemical weapons by the Syrian government.

Mark Lyall Grant told reporters Tuesday the evidence Britain has suggests Syria has used different chemical agents and a combination of agents in some cases, at times including sarin.

"It is relatively small quantities but nonetheless repeated use, and any use of chemical weapons is abhorrent," he told a press conference.

Britain has reported a number of incidents to UN Secretary-General Ban Ki-moon and the investigation team he appointed, including three further incidents on May 23.

UN spokesman Martin Nesirky said Ban finds the "catalogof atrocities" in the UN report "sickening and staggering."