THE HAGUE—The on-time destruction of Syria's chemical weapons will depend on security cooperation and temporary cease fires by both sides in the country's civil war, the head of the agency overseeing the operation said on Wednesday.

"I wouldn't say unrealistic, but much depends on the situation on the ground," Ahmet Uzumcu, director-general of the Organization for the Prohibition of Chemical Weapons, said of the timeline set by the U.S. and Russia. "The safety and security of our experts is an overriding concern."

Under the U.S.-Russia agreement, ratified by the OPCW and the United Nations Security Council, Syria's chemical-weapons production, mixing and filling facilities must be destroyed by Nov. 1, faster than inspectors typically operate. The country's entire chemical stockpile must be destroyed within the first half of next year.

Free Syrian Army fighters rush to help a fellow rebel after he was shot byasniper in Aleppo's Salaheddine neighborhood on Wednesday. Reuters

"If we can ensure some cooperation by all parties, and if some temporary cease-fires could be established to permit our experts to work in a permissive environment, I think our targets can be reached," Mr. Uzumcu said.

Discussions of such possible cease fires are under way at the United Nations in New York, officials said.

The OPCW was launched in 1997, when the Chemical Weapons Convention took effect. The Convention has 189 signatories. Syria applied to be the 190th after the U.S.-Russia agreement was reached last month under the threat of possible U.S. military attack on the regime of Syrian President Bashar al-Assad.

Inspectors from the organization have visited one of the more than 20 sites they need to scrutinize. A visit to a second site was scheduled for Wednesday. Initial destruction of some warheads, bombs, and mixing-and-filling equipment began Sunday.

Fifteen inspectors from the OPCW's initial team that entered Syria on Oct. 1 remain in the country. They were supplemented on Wednesday by another team of 12.

Officials declined to say how many inspectors would ultimately be needed to supervise the destruction of the chemical arsenal in Syria, other than to say it will be in the tens rather than the hundreds.

OPCW activities so far are focused on destroying the production equipment. After that, when the chemical stockpile itself is being destroyed, the amount of manpower required will jump significantly.

"When you get into destruction verification, our inspectors have to verify every moment of destruction activity. That consumes a lot of inspector days," said OPCW spokesman Michael Luhan. "You have to stay on site 24-7. A lot of chemical-weapons-destruction facilities operate around the clock."

The OPCW is scheduled to adopt a series of specific milestones for the chemical-weapons destruction on Nov. 15. But the broad timeline initially set by the U.S. and Russia is far faster than the agency is used to working.

Adding to the challenge, OPCW inspectors are for the first time conducting their work in a war zone.

"It's unprecedented and we're at the beginning of a difficult process," Mr. Uzumcu said. He added that the OPCW is well-equipped to handle the job.

Syrian authorities are responsible for providing security for the OPCW team, with additional protection supplied by U.N. officials. If that proves insufficient, the inspectors will have to refuse to visit certain sites, officials said.

"At the moment there are certain sites located in areas which are dangerous," said Malik Ellahi, Mr. Uzumcu's political adviser. "We haven't started our inspection work in those sites yet, but soon we will be required to do so. At that point, it will be necessary for us to be assured that conditions on the ground are such that our teams can proceed to do their work in safety."

Several countries have given assistance to the OPCW mission, including Germany and Italy, which have provided transportation for the inspectors.

Syria's formal declaration of the details of its chemical-weapons arsenal is due by Oct. 27. The regime of Bashar al-Assad submitted an initial disclosure on Sept. 19 and sent further material last week.

The documents "look promising," the OPCW said, while adding that "further analysis, particularly of technical diagrams, will be necessary, and some more questions need to be answered."

Mr. Uzumcu on Wednesday echoed the comments of other OPCW officials that the Assad regime had been cooperative. The Syrians have appointed a liaison who is currently in The Hague, where the OPCW has its headquarters.

Under the international treaty that governs the destruction of chemical weapons, Syrian authorities are responsible for carrying out the destruction of their own program. OPCW inspectors are charged with supervising and verifying that destruction.

Syria is estimated to have 1,000 tons of chemical weapons. The stockpile consists largely of precursor materials, so the preferred method of destruction will be to use neutralizing agents, rather than the incineration that can be used for fully weaponized chemicals.

While some productions facilities can be converted to peaceful uses, those are in the minority.

"In most cases, chemical weapons facilities have to be not only destroyed, but erased from the face of the earth," said Mr. Luhan, the OPCW spokesman.

Write to Naftali Bendavid at