The Syrian government missed the December 31 deadline to transfer the first shipment of its chemical arsenal to international forces, as part of an agreement brokered by the US and Russia to rid Syria of the country’s lethal stockpile by June 2014.

Norwegian and Danish ships departed as planned from Cyprus on Monday and headed toward the Syrian coast. However, the vessels returned to dock and refuel in Cyprus on Tuesday after the chemical weapons — slated for transport to the northern coastal city of Latakia, Syria, under Chinese and US surveillance — failed to arrive.

“We are still on high alert to go into Syria,” the Norwegian ship HnoMS Helge Ingstad’s spokesman Lars Hovtun told the AFP news agency. “We still don’t know exactly when the orders will come.”

Reasons for the delay include security considerations, bad weather, and road closures, according to a statement from UN’s Organisation for the Prohibition of Chemical Weapons from December 28.

“A number of external factors have impacted upon timelines, not least the continuing volatility in overall security conditions, which have constrained planned movements,” the statement said. ”Logistical challenges coupled with inclement weather have contributed to this delay.”

Under the original timeline, the first batch of chemical weapons — reportedly including approximately 20 tons of mustard nerve agent — was to be moved from various storage locations in Syria to Latakia and transferred to Nordic and Danish vessels by December 31. The warships were then slated to transport the cargo to the US Maritime Administration vessel MV Cape Ray at an Italian port, and would subsequently be destroyed by US forces at sea.

In spite of the missed deadline, the spokesman for OPCW remained optimistic that the destruction of the arsenal would be completed and stressed that progress has been made in disarming Syria from its chemical capabilities.

“An enormous amount of work has been accomplished in three months,” OPCW spokesman Christian Chartier told AFP. ”Syria’s chemical arsenal has been completely neutralized, the chemical agents and chemical products are under international control, have been sealed… The effective dismantling of the production and filling plants is on course.”

The OPCW has been directed by the United Nations to oversee the destruction of the Syrian government’s chemical weapons. The unprecedented disarmament in the midst of a civil war, now in its third year, was launched following an August 21 chemical weapons attack on a Damascus suburb that killed hundreds of civilians.

The US and Western allies accused the Syrian government of being responsible for that attack, while Damascus blames the rebels. Syria joined the OPCW and agreed to dismantle its chemical arsenal to ward off possible US military strikes.

The Associated Press contributed to this report.