The UN says that one of its workers has been killed and others injured after a compound in Gaza was damaged on Wednesday, adding that the circumstances of the incident remain unclear.

The Palestinian territory’s Hamas-run health ministry blamed an Israeli strike and said five critically injured foreign workers had arrived in hospital. Israel’s military denied striking the UN compound in Deir al-Balah.

It comes after Israel said it was resuming fighting in Gaza following a two-month ceasefire – launching a wave of strikes that killed more than 400 people, according to Gaza’s health ministry.

On Tuesday, Israel’s Prime Minister Benjamin Netanyahu said it had “resumed combat in full force”.

A day later, the Israel Defense Forces (IDF) said it was extending ground operations in Gaza up to the Netzarim corridor, which divides the north and south of the strip.

Troops moved into the area “to create a partial buffer between northern and southern Gaza”, the military added.

The UN initially said two of its workers had been killed but later clarified that the second person was not a staff member.

The UN Office for Project Services (UNOPS) said an “explosive ordnance was dropped or fired” at the building, which was in an “isolated” location.

It added there was no confirmation on the nature of the incident or the type of artillery used.

UNOPS executive director Jorge Moreira da Silva described the incident as “not an accident” and added that “UN personnel and its premises must be protected by all sides”.

In a statement, a spokesperson for UN Secretary General António Guterres said two guesthouses were hit and called for a full investigation.

Footage verified by the BBC showed injured people – two still wearing blue UN flak jackets – arriving at a hospital in an ambulance and a UN car.

Separately, at least 20 people were killed in air strikes across Gaza overnight, after Israel said it was resuming fighting in the Palestinian territory.

Two civilians were killed and five others injured when an Israeli drone hit a tent near the al-Mawasi humanitarian zone, the Palestinian Wafa news agency reports, citing Red Crescent medics.

Israel’s army said it had targeted what it called a Hamas military site, from where the group was preparing to fire into Israel. Vessels controlled by Hamas were also hit, the army said.

The bombing is not of the same scale as it was on Tuesday – but it shows no let-up in Israel’s fresh assault.

Tom Fletcher, the UN’s under-secretary-general for humanitarian affairs and emergency relief, said of Tuesday’s strikes that “the intensity of the killings is now off the scale”.

Wafa says a woman and child were killed in an air strike north of Khan Younis overnight into Wednesday, while four others were killed in a strike in Gaza City.

Gaza’s health ministry said that 436 people had been killed in strikes since Tuesday, including 183 children.

Tuesday’s strikes constituted the heaviest bombardment since a fragile ceasefire and hostage exchange deal came into effect on 19 January, and came after Israel and Hamas failed to agree how to take it beyond an initial phase.

The deal involves three stages, and negotiations on the second stage were meant to have started six weeks ago – but this did not happen.

Under the proposed second phase, Israel would withdraw troops from Gaza – but Israel and the US instead pushed for an extension of the first phase, with more hostages being released in exchange for more Palestinian prisoners.

Netanyahu cast the resumption of fighting as a return to Israel’s primary aims – to return the hostages and “get rid” of Hamas – but families of hostages have criticised the decision, saying it showed the government had given up on their loved ones.

Thousands of Israelis have joined a protest in Jerusalem, accusing Netanyahu of undermining democracy and restarting the offensive in Gaza without regard for the hostages.

Israel says Hamas is still holding 59 hostages, 24 of whom are believed to be alive.

Egypt, a mediator in talks, said the fresh strikes were a “blatant” violation of the ceasefire.

Israel previously imposed a total halt on all humanitarian aid entering Gaza, causing widespread international alarm.

“For two weeks now, our food supplies are rotting at the borders, the medicines are expiring, the water’s been cut off, the power’s been cut off – and all that to punish civilians further,” Mr Fletcher told BBC Radio 4’s Today programme.

He said he had spoken to the UN security council on Tuesday in a bid to lift the blockade and get the ceasefire deal back on track, including the release of hostages.

“I’m not asking for the moon here,” the UN chief added.

He said his team would “carry on going” in Gaza because they are “determined to do everything they can to save as many survivors as possible.

“They’re saying to us: What does it say about our values that we can’t stop a 21st Century atrocity happening before our eyes – and not just happening, but being cheered on before our eyes?”

Hamas has confirmed that several of its leaders were killed in Tuesday’s strikes, including its de facto head of government, Essam a-Da’lees.

Meanwhile, Islamic Jihad – whose fighters participated in the 7 October 2023 attack which triggered the current conflict – said the prominent spokesman of its armed wing, known as Abu Hamza, was killed.

However, there were also many civilians, including dozens of children, thought to be among the dead.

Regional mediators are now said to be pushing Hamas to release some of the Israeli hostages it still holds in exchange for a de-escalation.

But Netanyahu has said that going forward, all ceasefire talks will take place “under fire”.

The 7 October 2023 attack by Hamas on Israel saw about 1,200 people killed and the capturing of 251 hostages – 25 of whom were released alive during the first phase of the ceasefire.

Israel responded with a massive military offensive, which has killed more than 48,500 Palestinians, the Hamas-run health ministry says, as well as causing large-scale destruction to homes and infrastructure.