THE HAGUE (AFP) - The world's chemical watchdog called on Wednesday for Syria to speed up operations to hand over its arsenal for destruction, after missing a key deadline.

"We are exhorting the Syrian government to intensify its efforts, so we can conclude this critical part of the mission absolutely as fast as the conditions allow," Organisation for the Prohibition of Chemical Weapons spokesman Michael Luhan said.

The joint OPCW-UN mission said on Tuesday that a first cargo of chemicals had been brought to Syria's port of Latakia and transferred to a Danish vessel, although all Syria's most dangerous chemicals were supposed to have left the country by Dec 31.

So-called Priority One chemicals including mustard gas must all be destroyed by March 31.

A source close to the matter said that the OPCW's Executive Council, which is meeting on Wednesday, had noted progress -"notwithstanding the technical difficulties resulting in delays" - and encouraged "the Syrian Republic and all States parties to maintain the momentum".

Escorted by Chinese, Danish, Norwegian and Russian naval vessels, the Danish ship will stand offshore until more chemicals arrive at Latakia and then return to collect them.

Syria agreed last year to a US-Russian deal to hand over its chemical weapons.

That came after US President Barack Obama threatened air strikes following an August chemical weapons attack outside Damascus which killed hundreds of people, and which Washington blamed on the Syrian regime.

Under the plan, the chemicals will be transported to an Italian port and offloaded onto a US vessel, where they will be destroyed at sea.

The mission has a June deadline to complete the destruction of all of Syria's chemical weapons.

More than 130,000 people have been killed in the Syrian conflict since March 2011.