GENEVA — The head of the international watchdog overseeing the destruction of Syria’s chemical weapons arsenal said Wednesday that a team of 15 inspectors had begun to visit sites and that the Syrian authorities had been “quite constructive” and “cooperative.”

The official, Ahmet Uzumcu, director-general of the Organization for the Prohibition of Chemical Weapons, said completing the work within the “extremely tight” deadline of mid-2014 agreed on by the United States and Russia would depend on whether temporary cease-fires could be arranged between government and opposition forces.

“If we can ensure cooperation by all parties, and if some temporary cease-fires could be established in order to permit our experts to work in a permissive environment, I think the targets could be reached,” Uzumcu said, delivering his first public comments on the issue at a news conference streamed live from the organization’s headquarters in The Hague.

Inspectors, supported by U.N. personnel, have already visited one site and were set to visit another, said Uzumcu, a Turkish diplomat who has led the organization for several years. He noted that they would visit a total of around 20 in coming weeks.

The sites would be those identified by the Syrian government in a preliminary inventory of its chemical weapons program presented to the OPCW last month. The disclosures were made to meet the requirements of a U.N. Security Council resolution that warded off a U.S. military strike after an August poison gas attack in the suburbs of Damascus that killed hundreds of people.

U.S. officials had estimated that Syria had at least 45 chemical weapons sites before it started moving munitions and consolidating to increase security.

U.S. officials have said that, while the Syrian government’s preliminary inventory was more extensive than some anticipated, it was not complete. “Whether there has been sufficient cooperation is an open question,” a European diplomat in The Hague said Wednesday in a telephone interview, speaking on the condition of anonymity in line with diplomatic practice.

The Syrian government is to submit a more formal declaration by Oct. 27, which will be scrutinized for holes. “That will give us some ability to say how serious they are,” the diplomat said.

The declaration is expected to lay out the full history of Syria’s chemical warfare program, providing details on all sites involved in researching, producing or storing its arsenal. It is also supposed to lay out a plan for completing destruction, which will be monitored by the international inspectors.

The first team of inspectors from the OPCW arrived in Damascus last week. By Sunday, the team had reported that it was already overseeing the destruction of “Category 3” weapons — meaning missile warheads, aerial bombs and systems for delivering chemical agents as well as equipment for mixing and producing them — a process that is to be completed by the end of this month.

Underscoring the speed with which the program is unfolding, a second team of around 12 inspectors was expected to arrive in Damascus on Wednesday.

“The teams have jelled very well,” Michael Luhan, a spokesman for the organization, said in a telephone interview. “We’re happy with the way things are moving.”