The world's chemical weapons watchdog on Wednesday said all effluents from Syria's neutralised chemical weapons arsenal have been destroyed.

"The Organisation for the Prohibition of Chemical Weapons welcomes the disposal of effluents resulting from neutralisation operations aboard the US vessel Cape Ray," the Hague-based watchdog said in a statement.

A total of 1,300 metric tonnes of chemical weapons have been removed from Syria, with the majority neutralised on the US Navy ship MV Cape Ray -- and turned into less harmful effluent.

This effluent is the result of the sea-based destruction of sulphur mustards, commonly known as mustard gas, and methylphosphonyl difluoride, a precursor chemical for sarin gas.

Syria's chemical weapons stockpile was destroyed after President Bashar al-Assad's regime agreed to an international plan, following a 2013 sarin attack on a Damascus suburb that sparked a global outcry.

The United States threatened military action against Damascus over the attack, but held off following the disarmament agreement.

The OPCW said a waste disposal facility in Finland last Thursday said it had destroyed 5,463 metric tonnes of methylphosphonyl difluoride effluent.

On Friday, a German waste disposal firm destroyed some 335 metric tonnes of sulphur mustard effluent in Germany.

The German disposal has been verified by an OPCW team, while another verification team will shortly be sent to Finland, the watchdog said.

"This is yet another milestone in the path to eliminating chemical weapons stocks in Syria," OPCW director Ahmet Uzumcu said in the statement.

Of the 1,300 metric tonnes of Syria's declared chemical weapons only 16 metric tonnes of hydrogen fluoride remain to be destroyed at a facility in Port Arthur in Texas, the OPCW said.