Correction: Earlier versions of this article misstated the city where the Organization for the Prohibition of Chemical Weapons is headquartered.

Syria has destroyed its declared chemical-weapons production facilities, international inspectors said Thursday, marking a major step in the complex task of ridding the country of the weapons of mass destruction.

The declaration came a day before a Nov. 1 deadline as the team overseen by the inspectors hewed to an ambitious schedule for destroying Syria’s entire chemical arsenal by the middle of next year — a far more rapid process than comparable efforts in other countries and one that must be implemented in the middle of a civil war.

Weapons experts described the declaration as a milestone but warned that hurdles remain in the more challenging work of destroying the toxic agents and their precursor chemicals, some of which is expected to take place overseas. Syria has declared 1,290 tons of chemical agents and precursors, according to the Netherlands-based Organization for the Prohibition of Chemical Weapons, which is tasked with overseeing the destruction process.

Inspectors “confirmed today that the government of the Syrian Arab Republic has completed the functional destruction of critical equipment for all of its declared chemical weapons production facilities and mixing/filling plants, rendering them inoperable,” the OPCW said in a statement.

The process began in September, when Russia brokered an accord under which Syria agreed to surrender its chemical stockpile after the Obama administration threatened military action against the government of Syrian President Bashar al-Assad. The threat was in response to deadly attacks in August, apparently using sarin gas, on civilians in rebel strongholds outside Damascus.

Timeline: Unrest in Syria

Two years after the first anti-government protests, conflict in Syria rages on. See the major events in the country's tumultuous uprising.

Destruction of the mixing equipment and unfilled missile warheads began in the first week of October, starting with relatively simple items such as high-speed saws and blowtorches.

“Destruction of the equipment may have been the easy bit,” said Karl Dewey, a proliferation analyst at IHS Jane’s, a defense and security consultancy. “As it stands, the country’s chemical weapons stockpiles remain and need to be destroyed. A host country for this to happen in needs to be found. Transport and safety issues need to be addressed, particularly given the wider security concerns.”

About 1,000 of the 1,290 tons of the declared stockpile, which is in 41 facilities at 23 sites, consists of chemical precursors, according to a report that the OPCW submitted to the U.N. Security Council a week ago. The remainder consists of “category 2” — or weaponized — chemical weapons. The Syrian government has also submitted information on 1,230 unfilled munitions and says it found two cylinders that did not belong to it containing chemical weapons, the report said.

OPCW and European security officials have said they are confident that the Syrian government’s declaration of its stockpile is relatively complete. U.S. security officials, however, have said they are trying to resolve discrepancies between that declaration and Western intelligence estimates of the number of Syrian chemical weapons sites.

“This is a process that has to be based on mutual trust,” OPCW spokesman Christian Chartier said Tuesday. “How can you be sure nothing has been hidden? You can’t. But any member state of OPCW is at liberty to share with us intelligence or suspicions of the existence of additional sites, in which case we have the capacity to launch a visit within a very short time frame.”

Others remain more skeptical.

“History doesn’t give us many grounds for much optimism,” Dewey said. “Many past declarations — even those made under some kind of duress — have turned out to be less than accurate, and these sites have been declared by the Assad regime. There are suspicions that these lists may not be complete and/or a small capability may be retained.”

Inspectors visited 21 of the 23 sites that the Syrian government had declared in September as part of its accession to the Chemical Weapons Convention. They oversaw the work to render production facilities inoperable over the past month.

The two sites that the inspectors did not visit are in contested areas that the Syrian government does not fully control. But the OPCW said Thursday that government officials told it that both sites had been abandoned and their equipment and chemicals moved to the other sites. It was not possible to independently to verify that claim. The joint OPCW-U.N. mission is scheduled to produce plans for the destruction of the chemical agents by Nov. 15.

The opposition has complained that the plan to rid Syria of its chemical weapons buys the Assad government more time and allows it to continue killing by other means with impunity.

“Even if the chemical weapons are destroyed, what effect would that have on the ground?” said Aya Mahaini, an activist from the Damascus suburb of Moadamiya, which was hit in the Aug. 21 attack. “We have been killed for the last three years with all different kinds of weapons — tanks, mortar shells, snipers. Destroy the chemical weapons, or don’t. That won’t matter, because with chemical weapons or without them, the killing continues, the rivers of blood continue.”

Birnbaum reported from Berlin. Ahmed Ramadan in Beirut contributed to the report.