GAZA CITY ( CNN ) -- Gaza 's main hospital , already overloaded with Palestinians wounded in the week-long Israeli air assault , has reached critical mass , according to a Norwegian doctor volunteering at Shifa Hospital . A Palestinian father carries his wounded baby daughter into a hospital in Gaza City Sunday . `` The injured patients are mainly civilians , a lot of children with dreadful injuries , '' Dr. Erik Fosse told CNN on Monday , estimating that 20 percent of the more than 500 people dead were children . `` This figure is rising , and I think it has to do with the development of the war as it moves into the city , '' he added . After a weeklong series of air strikes , Israel launched a ground assault Saturday night . Palestinian medical officials said Israeli forces have killed 37 Palestinians -- both civilians and militants -- since moving into the territory . With those deaths , at least 507 Palestinians have been killed in the military operation , including about 100 women and children , officials said . In addition , 2,600 Palestinians have been injured , most of them civilians , officials said . `` We 've had a steady stream ( of patients ) every day , but the last 24 hours has ( been ) about triple the number of cases , '' Fosse said late Sunday . Fosse said he estimated that about 30 percent of the casualties at Shifa -- Gaza City 's main hospital -- on Sunday were children , both among the dead and wounded . The increase in casualties at Shifa followed Israel 's ground incursion into Gaza . Fosse said 50 patients were `` severely wounded '' when an Israeli air strike hit a food market in Gaza City . Watch Palestinians describe fearful life in Gaza » . `` We were operating in the corridors , patients were lying everywhere , and people were dying before they got treatment , '' he said . Most of the casualties were due to the air strikes that preceded the ground incursion . Other hospitals in Gaza could not treat the wounded because of a shortage of supplies and staff . Israel has said the military operation is a necessary self-defense measure after repeated rocket attacks from Gaza into southern Israel by Hamas militants . Israeli leaders say they are trying to minimize civilian casualties in Gaza . Last week , Dr. Eyad El-Sarraj , a psychiatrist who runs Gaza 's mental health program , said Gaza was headed for `` a major humanitarian disaster '' unless the fighting ended soon . Meanwhile , at the Gaza-Egypt border , nearly 25 trucks carrying aid and medical supplies were unable to pass through the Rafah border crossing on Sunday , CNN 's Karl Penhaul reported . Egyptian authorities said the guards who were manning the Palestinian side of the border had abandoned their posts . Aid workers and drivers banged on the gate to protest the closure . Watch `` absurd '' situation at border crossing » . An official with the humanitarian group World Vision also confirmed that report , saying : `` Unfortunately today , they closed the border , so no aid entered Gaza today . '' `` There are food shortages ... The health system is overwhelmed . The people here do n't have electricity , '' added Mohammed El-Halaby , program manager for World Vision , adding that several power lines and water pumps were damaged by last week 's air strikes . On Saturday -- before Israel launched its ground incursion -- old Palestinian ambulances had carried the wounded across the border , where patients were loaded into modern ambulances . Most of those taken into Egypt were civilians , including a teenage boy with his arm blown off , as well as a 4-day-old baby , who was not injured but needed to be kept on a ventilator and in an incubator . iReport.com : Share reactions to the crisis in the Middle East . About 10 truckloads of donations from Saudi Arabia , Qatar , Turkey and Greece crossed into Gaza on Saturday .
NEW : Doctor : Some 20 percent of the 500 people dead at hospital are children . Comment: : At least 507 Palestinians have been killed in the military operation . Doctor : `` people were dying before they got treatment '' Some 25 trucks bearing aid could not cross the Gaza-Egypt border on Sunday .