A new problem is bedeviling humanitarian aid convoys attempting to deliver relief to hungry Gazans: attacks by organized crowds seeking not the flour and medicine that trucks are carrying, but cigarettes smuggled inside the shipments.

In tightly blockaded Gaza, cigarettes have become increasingly scarce, now generally selling for $25 to $30 apiece. U.N. and Israeli officials say the coordinated attacks by groups seeking to sell smuggled cigarettes for profit pose a formidable obstacle to bringing desperately needed aid to southern Gaza.

The Israeli authorities closely scan everything that goes in and out of Gaza through Israeli-administered checkpoints. But the cigarettes have managed to slip through for weeks inside aid trucks, mostly through Kerem Shalom crossing into southern Gaza.

To evade Israeli inspections, smugglers — mostly in Egypt — have been hiding them in sacks of United Nations-donated flour, diapers and even a watermelon, according to aid agencies and an Israeli military official who shared photos with The New York Times.

Aid trucks that set off from the crossing into Gaza were then attacked by crowds of Palestinians, some of them armed, seeking the cigarettes hidden inside, according to U.N. and Israeli officials.

Andrea De Domenico, who runs the U.N. Office for the Coordination of Humanitarian Affairs in Jerusalem, confirmed that aid officials had “seen cartons of U.N.-branded assistance with cigarettes inside.” He said the contraband cigarettes had created “a new dynamic” of organized attacks on aid convoys.