By Dominic Evans

BEIRUT (Reuters) - Syria has destroyed or rendered inoperable all of its declared chemical weapons production and mixing facilities, meeting a major deadline in an ambitious disarmament program, the international chemical weapons watchdog said Thursday.

The Organisation for the Prohibition of Chemical Weapons (OPCW), which won the Nobel Peace prize this month, said its teams had inspected 21 out of 23 chemical weapons sites across the country. The remaining two were too dangerous to reach for inspection but the chemical equipment had already been moved to other sites that experts had visited, it said.

Syria "has completed the functional destruction of critical equipment for all of its declared chemical weapons production facilities and mixing/filling plants, rendering them inoperable," it said, meeting a November 1 deadline for the work.

The next target date is November 15, by when the OPCW and Syria must agree to a detailed plan of destruction, including how and where to destroy more than 1,000 metric tonnes of toxic agents and munitions.

Under a deal brokered by Russia and the United States, Damascus agreed to destroy all its chemical weapons after Washington threatened to use force in response to the killing of hundreds of people in a sarin attack on the outskirts of Damascus on August 21.

It was the world's deadliest chemical weapons incident since Saddam Hussein's Iraqi forces used poison gas against the Kurdish town of Halabja 25 years ago.

The United States and its allies blamed the forces of President Bashar al-Assad for the attack and several earlier incidents. Assad has rejected the charge, blaming rebel brigades.

"This was a major milestone in the effort to eliminate Syria's chemical weapons program," Ralf Trapp, an independent chemical weapons disarmament specialist, said.

"Most of the sites and facilities declared by Syria to the OPCW have been inspected, their inventories verified, equipment for chemical weapons production disabled and put beyond use, and some of the unfilled weapons have also been disabled."

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Assad still has a substantial conventional arsenal. Israel declined comment on Thursday on reports its warplanes had struck a military base where Syrian opposition sources said his forces had stored powerful, Russian-made missiles.

WORKING IN WAR

The OPCW mission is being undertaken in the midst of Syria's 2-1/2 year civil war, which has killed more than 100,000 people. There had been concerns that the violence would impede the disarmament, but the OPCW says Syrian authorities have been cooperating with the weapons experts.

At one location it could not visit, the OPCW said it was able to verify destruction work remotely, while Syrian forces had abandoned the two sites it could not inspect at all.

Syrian authorities said that "the chemical weapons program items removed from these sites were moved to other declared sites", an OPCW document said. "These sites holding items from abandoned facilities were inspected."

Trapp said it was "important to ensure that the remaining facilities can be inspected and their equipment and weapons inventoried and prepared for destruction as soon as possible".

Amy Smithson, a chemical weapons expert at the U.S. Monterey Institute, cautioned that the work achieved so far had been relatively easy compared with the next stage, which will involve transporting and eliminating warfare agents.

The OPCW also remained reliant on goodwill from Damascus, said Smithson, noting that Saddam and late Libyan leader Muammar Gaddafi had tried to pull the wool over inspectors' eyes in the past.

"What is unknown at present is whether Assad has declared everything in his arsenal - remember, Gaddafi kept a stash and Saddam tried his best to do the same but was outmaneuvered by savvy, determined inspectors - and to what extent Syrian cooperation will continue," she said.

Under the disarmament timetable, Syria was due to render unusable all production and chemical weapons filling facilities by November 1. By the middle of next year it must have destroyed its entire stockpile of chemical weapons.

The OPCW has not said which locations it had been unable to inspect, but a source briefed on their operations said one was at Safira, southeast of Aleppo in the north of the country. The site itself remains under government control but has been emptied of equipment because of fighting nearby.

The Syrian Observatory for Human Rights, which monitors the violence in Syria, said forces loyal to Assad advanced towards Safira on Thursday and clashed with rebels on its eastern approaches.

The other site not inspected was at Tel Kurdi in Adra, northeast of the capital Damascus. Tel Kurdi is now under rebel control but has been empty since early 2013 when the equipment was moved to another site, the source said.

The site which was inspected remotely was Al Sukkar, also in Adra, which contained instruments, ammunition and other substances which were destroyed by Syrian officials. The OPCW monitored this operation by video because the site, although under government control, was dangerous to reach.

(Additional Reporting by Anthony Deutsch; Editing by Will Waterman and David Stamp)