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.”
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.
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.
A day later, the Israeli Defense Forces (IDF) relaunched ground operations, seizing part of a key corridor that divides the north and south of Gaza – the Netzarim corridor.
On Thursday, the IDF said it had begun a ground operation in northern Gaza.
Meanwhile, Hamas fired three rockets at Israel without causing casualties.
It was the first such attack since the resumption of the IDF’s strikes.
Palestinians mourn loved ones killed in Israeli strikes in Khan Younis. Pic: Reuters
Hospitals across Gaza said the strikes hit houses in Khan Younis and Rafah in southern Gaza and the northern town of Beit Lahiya.
The Israeli military said it was looking into the reports.
Palestinians inspect the site of an Israeli strike on a house, in Khan Younis in the southern Gaza Strip. Pic: Reuters
The European Hospital in Rafah said 26 people, mostly women and children, had been killed in strikes on two family homes overnight.
A father and his seven children were killed in one of the attacks, it added.
The Nasser Hospital in Khan Younis received the bodies of seven people killed overnight in an attack on a home, it said.
A child looks on as Palestinians inspect the site of an Israeli strike on a house, in Khan Younis. Pic: Reuters
Meanwhile, Hamas said talks with mediators were ongoing.
Fragile ceasefire
Hamas and Israel disagreed over how to proceed with the ceasefire after phase one came to an end.
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.
Read more:
What happened to the Israel-Hamas ceasefire?
Siblings among those killed in one of Gaza’s deadliest days
A man sits amid rubble in Khan Younis. Pic: Reuters/Hatem Khaled
Israel wanted to temporarily extend phase one and for more hostages to be released.
It cut off all supplies to Gaza to try and pressure the group into doing so before restarting its military campaign.
The war started after Hamas militants attacked Israeli communities in October 2023, killing 1,200 people and taking more than 250 hostages.
More than 49,000 Palestinians have been killed in the ensuing conflict, according to Gaza’s health authorities,
