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. Word of health facilities being pushed to the limit came as Israel announced on Monday it was opening up border crossings to allow the flow of humanitarian goods into the Palestinian territory. "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.
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NEW: Israel announced on Monday it was opening up border crossings
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NEW: It will allow 80 trucks filled with humanitarian supplies to pass into Gaza
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Doctor: Some 20 percent of the 500 people dead at hospital are children
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At least 507 Palestinians have been killed in the military operation
@placeholder leaders say they are trying to minimize civilian casualties in Gaza.