Fighting resumed in the Gaza neighborhood of Shujayea less than an hour after a "humanitarian truce" was agreed to allow the evacuation of wounded from the area, where dozens had been killed overnight.
Israel's military said its forces were shot at shortly after the two-hour truce, facilitated by the Red Cross, had begun at
1:30 pm (10:30am GMT) on Sunday, and that it had resumed combat operations.
Hamas had no immediate comment on Israeli allegations it had breached the ceasefire.
Robert Turner, UNRWA's director of operations in Gaza, told Al Jazeera that hospitals are running low on supplies and that the majority of the casualties being admitted are civilians, with many suffering shrapnel injuries.
Shortages of drugs, water, electricity and medical supplies were also reported.
More than 400 Palestinians have been killed since the beginning of Israel's "Operation Protective Edge" on July 8. Dozens of children are among the dead.
Israel launched the offensive in response to rockets fired from Gaza and says it is targeting military installations of Hamas and other armed groups.