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 al-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,” Mr. 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.