Syria has moved the first batch of chemical weapon materials out of the country after transporting it from two sites to the port city of Latakia and onto a Danish vessel, the international chemical weapons watchdog said on Tuesday.

"The vessel has been accompanied by naval escorts provided by Denmark and Norway, as well as the Syrian Arab Republic," the Organisation for the Prohibition of Chemical Weapons (OPCW) said in a statement. "It will remain at sea awaiting the arrival of additional priority chemical materials at the port."

Syria agreed to abandon its chemical weapons by June under a deal proposed by Russia and hashed out 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.

War, bad weather, bureaucracy and technical issues delayed a Dec. 31 deadline for the removal of the most deadly toxins from Syria. The OPCW did not say what percentage of the "most critical" chemicals, including around 20 tonnes of mustard nerve agent, were on the Danish vessel.

"A first quantity of priority chemical materials was moved from two sites to the port of Latakia for verification and was then loaded onto a Danish commercial vessel today," the OPCW statement said. It said that maritime security was being provided by Chinese, Danish, Norwegian and Russian ships.

The Syrian government is responsible for the safe packaging, transport along roads to Latakia - including the main highway from the capital where rebels are still active - and removal of chemical weapons.