BEIRUT, Lebanon — In the Syrian city of Aleppo, children carrying groceries climb 15-foot mounds of rubble on their way home. Shoppers ignore vibrations from falling bombs. Buildings stand sliced in half, wires and beds and bathtubs exposed, with families still living inside. Most days, doctors have just seconds to decide which children to try to save and which to let die, as parents shriek and explosions shake the ground.

Three American doctors said such scenes were replaying in their minds after a recent visit to the insurgent-held section of Aleppo. The doctors provided a fresh perspective on life there, because their Syrian colleagues in Aleppo, after four years of bombardment by government forces, have grown tired of describing the horrors to the outside world and have, in some sense, stopped noticing.

“For them, that’s just their life,” said Dr. Samer Attar, one of the American doctors from the Syrian American Medical Society who volunteered for two weeks in rebel territory, then raced for the Turkish border just as pro-government forces cut off the last route out.

Their mission was to relieve, and honor, exhausted medical workers; bear witness to the ordeal of civilians in Aleppo; and press the world to help them. But now, the doctors said, the emergency has grown even more dire. Since Thursday, eastern Aleppo has been completely encircled, bracing for a starve-or-surrender siege like the ones government forces have used to take back other rebellious areas, and intense bombing continues.