LONDON — Pressure intensified on Syrian rebels on Monday to permit access to chemical weapons sites in areas under their control, as officials said the rapidly shifting lines in the civil war made it difficult for inspectors to reach some locations and called for all parties to ease the process of dismantling Syria’s toxic arms.

A Western diplomat in the Arab world said that though the Syrian government was legally responsible for dismantling its chemical weapons under an international agreement, its opponents should also cooperate in the process, because 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 the condition of anonymity to discuss a delicate issue. “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 from the Organization for the Prohibition of Chemical Weapons, the watchdog group in charge of implementing the agreement along with the United Nations, has not publicly cited any specific instance of opposition fighters’ impeding access to chemical weapons sites. As with agencies that deliver relief aid, the inspectors face a complicated and uncertain process that requires cease-fires with multiple parties among fluid lines of combat.