Around 700 tonnes of Syria's chemical agents have been delivered to specialised destruction facilities in Britain, Finland and the US, the watchdog in charge of the mission said Thursday.

"All 1,300 metric tonnes of chemicals removed from Syria by the international maritime operation have been delivered to destruction facilities outside the country," the Organisation for the Prohibition of Chemical Weapons (OPCW) said in a statement.

The remaining around 600 tonnes of Syria's declared chemical arsenal is currently being destroyed aboard a US military ship at a secret location at sea.

"Destruction activities are now underway in all locations," the OPCW said.

A plan has also finally been agreed for the destruction of Syria's 12 former chemical weapons production facilities, the OPCW said.

Seven of them will be "razed to the ground and five underground structures will be sealed permanently to make them inaccessible," the OPCW said.

There had been concern that Syria could re-use its fortified facilities if they were rendered inaccessible rather than destroyed, given the ongoing conflict.

Destruction of Syria's former chemical weapon production facilities will begin within 60 days, the OPCW said.

Syria signed up to an international UN-OPCW plan to destroy its chemical weapons stockpile after the outcry that followed chemical attacks by the Assad regime in the suburbs of Damascus on August 23 last year, that may have killed as many as 1,400 people.