LONDON — 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 five 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,’’ 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 that requires cease-fires with multiple parties among fluid lines of combat.
The International Committee of the Red Cross, for instance, said Monday that four of the seven aid workers 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 personnel.
Highlighting the perils of working in Syria, activist groups there said a car bombing in a northwestern market town under rebel control in Idlib province killed at least 12 people Monday, the Associated Press reported. Some activists put the number of dead as high as 20.
Since it began as a crackdown on civil protest in March 2011, the war has claimed more than 100,000 lives. In recent days, car bombings appear to have become more prevalent, with two such attacks near the state television building in Damascus on Sunday.
Secretary of State John Kerry and the UN-Arab League envoy for Syria said Monday that an international conference to set up a Syrian transitional government must be held as soon as possible.
‘‘There has to be a transition government, there has to be a new governing entity in Syria in order to permit the possibility of peace,’’ Kerry said. Kerry and envoy Lakdhar Brahimi spoke to reporters after meeting in London.
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 US and French threats to launch retaliatory military strikes against targets in Syria.
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.”