Israeli forces have launched a “limited ground operation” to retake the Netzarim corridor, a newly widened road protected by fortified bunkers that divides Gaza and is seen as essential to controlling the devastated Palestinian territory.

The move is a significant escalation of Israel’s new offensive in Gaza and came less than 36 hours after a massive wave of airstrikes that killed more than 400, including 183 children and 94 women, the health ministry there said.

Zaher al-Waheidi, the head of the ministry’s records department, described those attacks as the deadliest day in Gaza since the start of the war.

More Israeli strikes on Wednesday killed about 20, bringing the death toll to 436 in 48 hours, a spokesperson for Gaza’s civil defence service said.

Among the new casualties was a UN staff member, who was killed when two UN guesthouses in Deir al-Balah in central Gaza were hit in an attack.

The UN has called for an investigation.

“The locations of all UN premises are known to the parties to the conflict, who are bound by international law to protect them and maintain their absolute inviolability,” a spokesperson for António Guterres, the UN secretary general, said.

The Israel Defense Forces denied hitting a UN building, saying “there was no IDF operational activity” in the area of the UN compound and “the IDF didn’t strike” it.

A series of Israeli evacuation orders telling about 150,000 people in the north and east of Gaza to leave their homes to avoid being trapped in a combat zone suggested ground assaults in coming days, but seizure of the Netzarim corridor is the first major move to retake territory in Gaza since Tuesday’s airstrikes, which shattered a two-month-long pause in the hostilities with Hamas.

A private security company that had been securing checkpoints in the Netzarim corridor withdrew overnight and Israeli troops in armoured vehicles and tanks and on foot moved in at dawn on Wednesday, according to western aid officials.

The Israeli forces are believed to have reoccupied four fortified bases there and closed off all access. Travelling from north to south Gaza is now impossible, the officials said.

Benjamin Netanyahu, Israel’s prime minister, has said the new offensive will continue until “total victory” is achieved over Hamas and the 59 remaining hostages held by the militant group are freed.

As part of the ceasefire deal agreed in January, Israel had withdrawn from the Netzarim corridor.

Separately, Israel Katz, the Israeli defence minister, threatened ordinary people in Gaza.

Katz issued a “last warning” to Palestinians in Gaza on Wednesday, saying in a video statement that they should follow the recent advice of the US president, Donald Trump, to return Israeli hostages and remove Hamas from power.

“Return the hostages and remove Hamas, and other options will open up for you – including the possibility of leaving for other places in the world, for those who want to,” Katz said.

Hundreds of thousands returned to what remained of their homes and Israeli forces pulled back to a buffer zone around the edges of Gaza during the ceasefire, which began in mid-January.

When the agreed first phase of the ceasefire ended 17 days ago, Israel reimposed a blockade on Gaza, cutting off all supplies.

The International Committee of the Red Cross (ICRC) warned on Wednesday that medics were struggling to manage a sharp increase in casualties over the last 36 hours after the resumption of ground operations by Israel in the Gaza Strip.

“Due to the recent suspension of humanitarian aid into Gaza, stocks of medical supplies have dropped significantly and on top of this, hospital staff are struggling to manage the sharp increase of casualties,” said the ICRC in a statement.

Israel and Hamas accuse each other of breaching the truce, which had broadly held since January and offered respite for the 2.3 million inhabitants of Gaza.

About 1,200 people, mostly civilians, died in the shock Hamas incursion into Israel in October 2023 that triggered the conflict. About 250 hostages were also taken. The Israeli offensive in Gaza has killed more than 49,000, also mostly civilians.

Netanyahu and other Israel officials said the decision to renew attacks in Gaza came after Hamas rejected proposals for a 30- to 60-day extension of the first phase of the three-phase ceasefire.

Hamas says it wants to conclude the phases of the ceasefire deal as signed.

Israel has refused to enter into discussions about the second phase, which is intended to lead to a permanent end to hostilities, the withdrawal of all Israeli forces from Gaza, and the return of all the hostages in Gaza, of whom more than half are thought to be dead.

“Hamas has not closed the door on negotiations, but we insist there is no need for new agreements,” Taher al-Nunu, a Hamas official, said on Wednesday.

Nunu called on the international community to “take urgent action” to end the war, while accusing Israel of “violating the ceasefire agreement it signed”.

Analysts said Israel had launched new strikes to “break an impasse”.

Prof Danny Orbach, an expert in military history at the Hebrew University of Jerusalem, said: “There was a very good reason Israel did not want to go to phase two [of the ceasefire agreement]. It would have meant Hamas staying in Gaza, staying in power and Israel having to lift its siege too … There was a complete disconnect between the interests of the two sides.”

Critics in Israel have accused Netanyahu of resuming the offensive to reinforce his coalition government before a crucial budget vote in parliament, to rally support for the war in the face of popular backing for a ceasefire, and to head off widespread public anger over his attempt to fire the head of the internal security service, the Shin Bet.

Underlining the deep divisions in Israel, tens of thousands in Israel protested on Tuesday night and Wednesday against the renewed offensive and Netanyahu’s government. More demonstrations are planned in coming days.