Israeli strikes killed at least 85 Palestinians across the Gaza Strip overnight and into Thursday, according to the Hamas-run health ministry.
Five UN staff members have also been killed, according to the head of the UNRWA.
It comes as fighting in the enclave has restarted in recent days.
Philippe Lazzarini, head of UNRWA, the United Nations Palestinian relief agency, said on Thursday: “In the past few days, another five UNRWA staff have been confirmed killed, bringing the death toll to 284.
“They were teachers, doctors and nurses: serving the most vulnerable.”
He added he feared “the worst is yet to come”.
“Under our daily watch, people in Gaza are again and again going through their worst nightmare,” he said in a statement.
“An endless unleashing of the most inhumane ordeals.”
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Speaking in Parliament, Foreign Secretary David Lammy said the UK government opposed the resumption of Israel’s hostilities and added that a British national was among the wounded when a UN compound was struck on Wednesday.
Israel has previously said it only targets militants and blames civilian deaths on Hamas because it operates in densely populated areas.
It denied previous reports it had targeted a UN compound.
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Briton wounded in Israeli strike
Israel resumes strikes
On Tuesday, Israel resumed heavy airstrikes on the enclave killing more than 400 people, bringing to a halt the ceasefire that had paused fighting in the 17-month war.
The militant group wanted to move onto phase two to negotiate Israel’s permanent withdrawal from Gaza, the release of hostages in exchange for more prisoners and an eventual end to the war.