White said the starvation and use of barrel bomb tactics have the effect of pacifying rebellious areas.
"It doesn't necessarily transfer them to full regime control, but for the regime it's working," he said.
One place where a tentative truce has been reached to allow in small, intermittent shipments of aid is the besieged Palestinian camp of Yarmouk in Damascus. The conditions there provide a window into the desperation weighing down all of the besieged areas.
The head of the U.N. relief agency that supports Palestinian refugees, Filippo Grandi, visited the camp this week, and described the haunting scene of emaciated and desperate people emerging from a cityscape of charred, blown-out buildings and gray, rubble-strewn streets to collect aid shipments.
"It's like the appearance of ghosts," Grandi said. "These are people that have not been out of there, that have been trapped in there not only without food, medicines, clean water — all the basics — but also probably completely subjected to fear because there was fierce fighting and noisy fighting going all along, and that was the most shocking point. They can hardly speak."
Associated Press writer Zeina Karam contributed to this report.