Evidence suggests chemical arms use in Syria

The report found "reasonable grounds" to believe that chemical weapons had been used on four occasions in March and April. The UN Commission of Inquiry could not ascertain precisely what types of agents were involved.

It called on Damascus to allow a team of UN chemical weapons experts into the country.

The report said the Syrian conflict had "reached new levels of brutality", with violations amounting to war crimes and crimes against humanity committed by both sides.

It comes as international powers struggle to set a date for a peace conference on Syria, where the conflict is believed to have cost at least 80,000 lives.

In the four months covered by this report - between 15 January and 15 May 2013 - investigators documented 17 possible massacres. It "documents for the first time the systematic imposition of sieges, the use of chemical agents and forcible displacement".

It said there were "reasonable grounds to believe that limited quantities of toxic chemicals were used" during attacks by government forces: on Khan al-Assal, Aleppo, 19 March; al-Otaybeh, Damascus, 19 March; Sheikh Maqsoud neighbourhood, Aleppo, 13 April; and Saraqeb, Idlib province, 29 April.

But it adds that it "has not been possible, on the evidence available, to determine the precise chemical agents used, their delivery systems or the perpetrator".