One strike in June, on a convoy of 15 trucks, killed five aid workers; another last month destroyed three trucks and forced the rest to turn back. Young men attempting to flee the city, scrambling over the berm at night, have been executed by fighters, Mr. Khater said. The road is lined with hastily dug graves and abandoned bodies, aid workers say. “They are just left there,” said Sylvain Penicaud, head of the Doctors Without Borders hospital in Tawila, by phone. The hospital treats about 40 sexual assault victims every week, but anecdotal evidence suggests “that is nothing compared to the true rate,” Mr. Penicaud said.