"The biggest bulk of the chemical weapons material is removed but not yet destroyed." Photo: BBC

Envoy Sigrid Kaag urged the Syrian government to meet a Sunday deadline to remove its arsenal from the country.

All Syria's chemical weapons are scheduled to be destroyed by 30 June.

The Russian-US deal to eliminate Syria's arsenal was drawn up last year after hundreds of people died in a sarin rocket attack outside Damascus.

The multinational mission to get rid of the weapons is overseen by the UN Security Council and the Organisation for the Prohibition of Chemical Weapons (OPCW).

"The biggest bulk of the chemical weapons material is removed but not yet destroyed and that counts towards the 30 June deadline. That's why it's so important to get the remainder of the chemical weapons material that is still in one site," Ms Kaag, the head of the OPCW, told the BBC.

She also said the UN was concerned by recent reports that Syrian forces had used chlorine gas as a weapon.

Chlorine was not a substance included in the deal, which is widely seen as having averted US military action against the Syrian government.

The US ship MV Ray has been specially fitted out for the destruction at sea of Syria's chemical weapons

Damascus has denied using chlorine gas as a weapon.

Most of Syria's chemical weapons substances exist as separate materials that only create the highly toxic warfare agents when mixed together, according to the OPCW.

Ms Kaag said the facilities need to produce, prepare and launch a chemical weapons attack had been destroyed.

"What remains are the elements of a chemical weapon, but the chemical weapons programme of Syria, as per the current declaration to the OPCW under the Chemical Weapons Convention is no longer in existence," she said.