Syria has surrendered almost two-thirds of its chemical weapons with the resumption of transfers from the war-torn country, the global chemical watchdog said Monday, although it again pressed Damascus to step up efforts.

"The Syrian government has completed the delivery of the 13th consignment of chemicals," the Organisation for the Prohibition of Chemical Weapons (OPCW) said.

"The deliveries have raised the overall portion of chemicals removed from Syria to 65.1 percent, including 57.4 percent of priority chemicals," it reported in a statement in The Hague.

Damascus had temporarily halted the transfer of its chemical stockpile, citing security reasons, but resumed the operations earlier this month.

Under the terms of the US-Russia brokered deal reached last year, Syria has until the end of June to destroy its chemical weapons if it wants to ward off the threat of US air strikes.

The agreement was reached after deadly chemical attacks outside Damascus last August that the West blamed on President Bashar al-Assad's regime.

Sigrid Kaag, who coordinates a combined UN-OPCW mission in Syria to oversee the transfer, told the UN Security Council on April 3 that Damascus could still make the June 30 cut-off.

But she warned any delay would make it "increasingly challenging" to stick to the deadline, diplomats in New York said.

OPCW chief Ahmed Uzumcu said the latest shipment out of Syria was "necessary and encouraging," but again warned that efforts had to be stepped up if the deadline was to be met.

In a statement, he said "both the frequency and the volumes of deliveries have to increase significantly" if the transfers are to be finished "against the projected time frame".

Norwegian as well as Danish naval vessels are involved in the process of removing the materials from the port of Latakia in western Syria, the most dangerous of which are to be transferred to a US Navy vessel specially fitted with equipment to destroy them at sea.