There is no doubt Syria's government has used sarin during the country's crisis, says France's foreign minister.

Laurent Fabius said lab tests in Paris confirmed numerous uses of the nerve agent, adding that those who resort to chemical weapons must be punished.

But he did not specify where or when the agent had been deployed; the White House has said more proof was needed.

Earlier, the UN said there were "reasonable grounds" to believe chemical weapons had been used.

In a new report, the UN commission of inquiry on Syria also urged foreign powers not to increase the availability of arms in Syria....

Mr Fabius said various samples taken from unspecified locations in Syria and tested in France showed the presence of sarin.

White House spokesman Jay Carney said: "We need to expand the evidence we have."

"There is no doubt that it's the regime and its accomplices" that were responsible, he told France 2 television in an interview.

Mr Fabius said the test results had been handed to the UN.

"All options are on the table," he added. "That means either we decide not to react or we decide to react including by armed actions targeting the place where the gas is stored."

The tests came after journalists for French newspaper Le Monde smuggled blood samples out of Syria after witnessing what they believed was a chemical attack in April.

Sarin, an extremely potent chemical nerve agent, is colourless and odourless. The use of chemical weapons is banned by most countries.

Both the Syrian government and the rebels have in the past accused each other of using the weapons.

The US was continuing its effort to gather evidence about the likely use of chemical weapons, the White House said on Tuesday....