Pressure mounted on Syrian rebels Monday to permit access to chemical weapons sites in areas under their control, as the head of the international watchdog on such toxic munitions said the rapidly shifting lines in the civil war made it difficult for inspectors to reach some locations.

Ahmet Uzumcu, director general of the Organization for the Prohibition of Chemical Weapons, which won the Nobel Peace Prize last Friday, told the BBC that the government of President Bashar Assad had been cooperating with inspectors who had reached 5 out of 20 chemical weapons production sites. But some other sites had "access problems," he said, reflecting perils and complexities facing inspectors who are trying to dismantle chemical weapons facilities as the war rages around them.

Some roads "change hands from one day to another, which is why we appeal to all sides in Syria to support this mission, to be cooperative and not render this mission more difficult," Uzumcu said. "It's already challenging."

A Western diplomat in the Arab world, moreover, said that while the Syrian government was legally responsible for dismantling its chemical weapons, its opponents should cooperate in the process, as several chemical weapons sites were close to confrontation lines or within rebel-held territory.

"The international community also expects full cooperation from the opposition," the diplomat said, speaking on condition of anonymity to discuss a sensitive issue.

There were clear signs from inspectors in Syria "that the government is delivering on its responsibilities and the opposition needs to hear a clear signal that they must play their part, too, in making sure that the inspectors have free and unhindered access to the chemical weapons sites with complete safety and security," the diplomat said. "However divided the opposition might be, it would look very bad if the government was seen to be cooperating fully, while inspections were held up because of problems with the opposition."

The inspection team has not publicly cited any specific instance of opposition fighters impeding access to chemical weapons sites. As with deliveries of humanitarian assistance, the inspectors face a complicated and uncertain process which requires cease-fires with multiple parties among fluid lines of combat.

The International Committee of the Red Cross, for instance, said Monday that 4 of 7 aid workers were abducted in northern Syria on Sunday - three of its staffers and a volunteer from the Syrian Arab Red Crescent - had been released. But there was no word on the other three abducted Red Cross personnel. 

The inspectors began arriving in Syria on Oct. 1 under an agreement brokered by the United States and Russia for Syria to dismantle its chemical weapons capability after a poison gas attack on Aug. 21 in a suburb of Damascus. Assad has denied accusations from the United States that Syrian government forces were responsible for the attack, which killed hundreds of people.

The agreement to destroy Syria's arsenal defused U.S. and French threats to launch retaliatory military strikes against targets in Syria in reprisal for the attack.

Uzumcu said inspectors from his organization, which is based in The Hague, had been so close to the fighting that mortar shells had exploded "next to the hotel where our team is staying, and there are exchanges of fire not far from where they go."
Last Friday, the Norwegian Nobel Committee awarded the peace prize to the organization, which was founded in 1997. The organization and the United Nations have a team of about 60 experts and support staff in Syria, the BBC reported.

Uzumcu said the Nobel award had come as "a very big boost of morale to them."
"They are working in very challenging circumstances in the field," he said. "In awarding the prize, they said it was about recognizing the work of the past 16 years, but also the work that lies ahead, in Syria."

The inspectors are facing a tight deadline set by the United Nations to complete their work by mid-2014.

In a report to the UN Security Council on Oct. 7, Secretary-General Ban Ki-moon acknowledged that the most difficult phase of work would start in November, when the teams of inspectors and UN personnel - a total of about 100 people - turn to the task of destroying an estimated 1,000 tons of precursor chemicals and weapons after disabling production facilities.

On Monday, Syria became the 190th member of the Organization for the Prohibition of Chemical Weapons by formally acceding to the 1997 Chemical Weapons Convention, which is intended to rid the world of such munitions. Officials at the organization said there would be no formal ceremony of accession.