Syria must "intensify efforts" to destroy its chemical arsenal in line with a timetable set by the United Nations, its disarmament mission chief told AFP Tuesday as a year-end target date was missed.

Sigrid Kaag said she could not specify a new date for Syria to complete shipping out the most dangerous components of its chemical weapons programme for destruction under international supervision.

But she said significant progress had been made despite the security and logistical problems that have plagued the disarmament effort, compounded by unusually severe snow storms earlier this month.

"We made it known that the deadline was unlikely to be met, however we've also made it known that continued solid progress has been achieved," said Kaag, who heads the joint mission of the UN and the Organisation for the Prohibition of Chemical Weapons (OPCW).

"And we're calling on all parties, including the Syrian Arab Republic, to intensify efforts needed to get to the point of the start of the removal of the chemical agents."

Under a timetable drawn up by the mission, Syria was supposed to have handed over key chemical components by Tuesday, although officials had made clear for a couple of weeks that that target was unlikely to be met.

Nordic naval vessels that had been deployed to escort the cargo ships carrying the chemicals for destruction off Italy returned to port in nearby Cyprus on Monday after they failed to reach the Syrian port of Latakia in time.

Kaag said the delay was the result of various obstacles and underlined the complexity of dismantling a chemical weapons programme in an active war zone.

"It is a complex operation, it requires a military component to secure convoys, it requires a lot of supplies to be in-country, prepositioned," she said.

"It requires a lot of preparation, to prepare the journey properly, to consolidate the chemical agents, to get them on trucks."

But she stressed: "At the end of the day, failure is not an option, success is within reach."

Kaag declined to set a new target date for the removal of the chemical materials from the country, saying that would be up to OPCW and UN members to decide.

But she said it was important to note the "benchmarks attained".

"I think we shouldn't confuse a deadline not being met on the day itself, versus all efforts and progress attained and the ability to be very close to a deadline to actually start implementation."

Syria agreed to turn over its chemical arsenal for destruction under the terms of a US-Russian deal enshrined in a UN resolution.

The agreement headed off military action threatened by the United States after a deadly chemical weapons attack on August 21 that reportedly killed hundreds of people outside the capital Damascus.

Western governments blamed forces loyal to President Bashar al-Assad for the attacks. He denied responsibility but agreed to join the Chemical Weapons Convention and turn over Syria's arsenal.

The deal calls for Syria's entire chemical arsenal to be destroyed by mid-2014, a target date that diplomats have acknowledged all along will be tough to meet as conflict rages daily between Assad's forces and the rebels.