At least 40 sites in Syria are associated with the nation’s chemical weapons program, and about half have significant amounts of materiel, a U.S. official told reporters Saturday in Geneva. Reports have indicated that some chemical stockpiles are situated around Damascus, the capital, while others may be found to the north, in Homs, Hama and Aleppo provinces, and to the west in coastal Latakia.

Still, even with the materiel concentrated in government-held zones, access will be problematic. No long-distance road in Syria is completely safe. And Syria’s armed rebels, who had been hoping for U.S. airstrikes that would degrade Assad’s forces, have generally rejected the U.S.-Russian deal, raising the possibility that chemical weapons inspection personnel could come under attack by opposition gunmen.

These days, the central core of Damascus is under extremely tight security, with military checkpoints every few blocks. Still, mortar shells launched from rebel-held suburbs regularly explode inside the city limits, killing civilians. Car bomb attacks are a regular threat.

Chemical weapons depots are unlikely to be in the densely populated center. Such facilities are more likely to be situated in the capital’s rambling outskirts, where vulnerability to rebel attack is much greater.