A top UN official has appealed to Syria to hand over urgently all remaining chemical agents after missing a deadline for their destruction, and urged Damascus’ allies to intervene.

Under a UN-backed and US-Russia brokered deal agreed last year after the United States threatened air strikes against Syrian government targets, the weapons were to be destroyed by June 30.

The deal was reached after a sarin nerve gas attack in a rebel-held Damascus suburb killed around 1400 people.

But 7.2 per cent of Syria’s declared chemical agents, though packed, remain inside the country, Sigrid Kaag, the UN official overseeing the process, told reporters in New York on Wednesday.

UN chief Ban Ki-moon had already confirmed in a letter to the Security Council that the June deadline would not be met and Kaag urged compliance as soon as possible.

She heads a joint mission by the United Nations and the Organization for the Prohibition of Chemical Weapons (OPCW) to disassemble the weapons.

“We call on all member states to exercise their influence to ensure the immediate removal of the remaining chemicals,” Kaag said after briefing the UN Security Council behind closed doors.

“The urgency, the time, the pressure to remove the remaining 7.2 per cent is very, very critical and I’ll be back in Damascus the next few days to pursue that conversation,” she added.

Asked if Syria was stalling, Kaag acknowledged security is a concern but warned: “It doesn’t mean that additional delays can be incurred.”

Danish and Norwegian ships are to take the chemicals from Latakia to a US ship for destruction at sea, as well as at sites in Finland, the US and Britain.