Syria has started moving chemical weapons materials out of the country in a crucial phase of an internationally backed disarmament programme that has been delayed by war and technical problems.

The joint mission overseeing the disarmament, the UN and the Organisation for the Prohibition of Chemical Weapons (OPCW), said on Tuesday that the materials had been moved from two sites to the port of Latakia and then loaded onto a Danish commercial vessel.

"This is an important step commencing the transportation of these materials as part of the plan to complete their disposal outside the territory of Syria,'' Mehmet Uzumcu, OPCW director-general, said in a statement.

"I encourage the Syrian government to maintain the momentum to remove the remaining priority chemicals, in a safe and timely manner, so that they can be destroyed outside of Syria as quickly as possible.'"

In separate interview with Al Jazeera, Sigrid Kaag, joint mission coordinator for the UN and OPCW, said the first shipment indicates the progress is being made.

"It is first in a series of moves that the Syrian authorities are expected to undertake to ensure that all chemical agents will be taken out of the country," Kaag said.

Syria agreed to abandon its chemical weapons by June under a deal proposed by Russia and agreed with the United States after an Aug. 21 sarin gas attack that Western nations blamed on President Bashar al-Assad's forces. Damascus blames rebels for the attack.

Washington welcomed the removal of chemical materials and said Assad's government appeared to be sticking to the deal.

"Much more needs to be done," State Department spokeswoman Jen Psaki told a news briefing, adding, "We have no reason to believe that the regime has gone back on any aspect of their promise."

China, Denmark, Norway and Russia are providing maritime security for the cargo.

The removal was due to take place before December 31, but Syria's worsening conflict, logistical problems and bad weather had delayed the operation.

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

Under the plan, the chemicals will be taken from the Syrian port of Latakia to a port in Italy, where they will be transferred to a US navy vessel, the Cape Ray, which is fitted with equipment to destroy them at sea.

The OPCW turned to the US military for assistance after no country volunteered to destroy the chemical weapons on its soil, despite an international consensus that the weapons be neutralised outside Syria.

The US-Russia deal was aimed at heading off US military strikes against President Bashar al-Assad's regime after hundreds of people were killed last August in a chemical weapons attack outside Damascus.