Danish and Norwegian naval vessels are to head for waters off Syria again Friday in readiness to escort a delayed shipment of key chemical weapons for destruction, a spokesman said.

The ships will weigh anchor from the nearby island of Cyprus, where they headed to port on Monday when it became clear that a year-end deadline for the shipment would be missed, Norwegian armed forces spokesman Lars Hovtun said.

Danish task group commander, Commodore Torben Mikkelsen, said no new date had yet been set for the shipment but that the vessels would hold station outside Syrian territorial waters in readiness for new orders.

"We don't know but what we know is that we are ready for the task and, if called upon, we are ready to be at short notice," Mikkelsen said in a video interview released by the spokesman.

When the operation gets underway, the ships are to be joined by Chinese and Russian vessels inside Syrian waters under a plan agreed in Moscow on Friday.

The year-end deadline for the removal of key weapons components was the first major milestone under a UN Security Council-backed deal arranged by Russia and the United States that aims to wipe out all of Syria's chemical arms by the middle of this year.

Syria's worsening civil war, logistical problems and bad weather had held up the operation to move chemical agents to the port of Latakia, said the joint UN-Organisation for the Prohibition of Chemical Weapons mission overseeing the operation.

Under the plan, the chemicals will be taken from Latakia to a port in Italy where they will be transhipped to a US Navy vessel specially fitted with equipment to destroy them at sea.