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 don'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.