Syria has submitted a "more specific" list of its chemical weapons to the global regulator overseeing the destruction of its stockpile after discrepancies were reported by inspectors on the ground, officials said.

Damascus agreed to give up its chemical arsenal after Washington threatened military action following the death of hundreds of Syrians in a sarin gas attack on the outskirts of Damascus during Syria's civil war last August. But Damascus is several weeks behind schedule in handing over its lethal toxins.

A diplomat said questions had been raised by member states at the Organisation for the Prohibition of Chemical Weapons (OPCW) about the details of its chemical arsenal submitted by President Bashar al-Assad's government last year.

The officials said the original list had been based on estimates, not exact amounts of toxic agents found in storage and production facilities across Syria.

The joint UN/OPCW mission in Syria found "discrepancies between what they found, and what was on the original declaration", one diplomat told Reuters.

OPCW spokesman Michael Luhan confirmed a revised list had been submitted. "For some of the stockpile, ranges of quantities had been provided. Now they are being replaced with specific amounts," he said.

The exact amounts came to light after inspectors visited the sites, took inventory, and packaged the chemicals for transport to the port town of Latakia, he said. Official could not provide specific details about the discrepancies.

Syria initially reported to the OPCW having roughly 1,300 metric tonnes of toxic chemicals, including precursors for poison gas and nerve agents. Luhan said no new chemicals were added in the revised list.

As part of a deal reached with the US and Russia last year, the Assad government agreed to abandon the weapons of mass destruction and destroy all chemicals in its possession by June 30.

Syria did not make public the exact list of chemicals, but officials have said it includes more than 500 metric tonnes of highly toxic chemical weapons, such as sulphur mustard and precursors for the poisonous gas sarin, as well as more than 700 metric tonnes of bulk industrial chemicals.

They are being loaded onto Norwegian and Danish ships in the Syrian port town of Latakia as part of a multi-million-dollar operation involving at least 10 countries.

The chemical weapons will be neutralised at sea on a specially-equipped US ship, the MV Cape Ray, while the bulk chemicals will be sent to commercial waste facilities in Finland, Britain and Germany.

But Syria has fallen several weeks behind schedule in handing over the chemicals, having shipped out nearly two thirds of the stockpile for destruction abroad.

After missing several deadlines, Syria submitted a revised plan to the OPCW, saying it would hand everything over by April 27, or within 10 days.

An official at the OPCW, who spoke on condition of anonymity, confirmed a new list had been submitted, but said it was part of a routine reporting process.

"Sometimes information is not complete, or not in a format we require. It's not extraordinary," the official said. "But what they have submitted needs to be seen to come to any conclusions and I better not speculate about what's in there."  - Reuters