"It is likely to be conservative and it is certainly rising," United Nations Relief and Works Agency spokesman Christopher Gunness said on Monday. He said the estimate was based on visits by UNRWA officials to hospitals and medical centers across the Gaza Strip.
Palestinian officials have said more than 300 people, including scores of civilians, have been killed in the Hamas-controlled territory since the Israeli offensive began.
Meanwhile, Israel opened the Kerem Shalom crossing on Monday morning in order to allow humanitarian aid – including food and medications – into the Strip.
Peter Lerner, a spokesman for the Coordination and Liaison Authority, said that the border crossing had been opened and that 80 trucks were expected to cross into Gaza.
The other border crossings, in Karni and Nahal Oz, remained closed for security reasons.
Nine hospitals in the Gaza Strip have collapsed under the burden of Israeli airstrikes. The Hamas Health Ministry announced that more than 800 people have been injured.
The Palestinian Center for Human Rights said that 251 Palestinians were killed by Sunday afternoon, including 20 children and youths under the age of 16 and nine women.
UN Secretary-General Ban Ki-moon on Sunday called for an immediate end to all violence in Gaza, which has been under Israeli assault for two days in retaliation for mortar and rocket attacks from the Hamas-ruled Palestinian territory.
The United Nations said Ban "deplores that violence is continuing today and he strongly urges once again an immediate stop to all acts of violence."
The UN Security Council earlier had called for an immediate end to all violence in Gaza. Ban underscored the council's call for Israel to open border crossings to Gaza to ensure its residents receive humanitarian supplies.
AFP contributed to this report