The officials say that while they and the Syrian Arab Red Crescent have been able to regularly reach many contested areas, in some places with growing food shortages they have routinely been blocked by either the government or the rebel groups, or both.

Hopes had been high last week that a United Nations convoy would reach the embattled rebel-held suburb of Moadhamiya for the first time in months, after an initial green light from some government agencies. But the trip did not materialize, United Nations officials said, after the government said military operations were continuing in the area.

The United Nations has also struggled to reach government-held parts of Aleppo, because the route from Damascus passes through areas controlled by myriad rebel groups. Many times, officials say, the convoys have been allowed through by a series of rebel factions, only to be stopped or looted by one farther along the road.

A pro-government Syrian journalist said he was shocked to learn that a number of chemical weapons facilities were in rebel-held areas. That, he said, may well mean that the dismantling process could easily take well over the yearlong timetable that has been proposed, and could require international forces to guard the inspectors and any weapons or materials that have to be moved.

That could draw international forces into direct conflict with rebel groups, something the journalist said could achieve a government goal: convincing the world that extremist groups among the rebels were the common enemy.

“There will eventually be a confrontation,” he said. “The idea of terrorism in Syria has been flowing in the media for a long time, but there was never a sense of direct threat to the West. It is there, yet in the long term, still for the West it is ‘the enemy of our enemy.' ” But, he added, “when you get to a point of actually confronting it, things change.”