Hundreds of thousands of civilians in Gaza remain trapped by the latest Israeli offensive centred on Jabaliya refugee camp, according to UN agencies and human rights groups.

“At least 400,000 people are trapped in the area,” Philippe Lazzarini, the head of the UN Palestinian refugee agency, Unrwa, posted on X on Wednesday, amid witness accounts of bodies lying uncollected in the streets because of the renewed fighting.

“Recent evacuation orders from the Israeli authorities are forcing people to flee again & again, especially from Jabaliya camp,” added Lazzarini. “Many are refusing because they know too well that no place anywhere in Gaza is safe.”

The Israeli military says the large-scale raid, now in its fifth day, is intended to stop Hamas fighters staging further attacks from Jabaliya and to prevent them regrouping.

Lazzarini said some Unrwa shelters and services were being forced to shut down for the first time since the war began and that with almost no basic supplies available, hunger was spreading again in northern Gaza.

“This recent military operation also threatens the implementation of the second phase of the #polio vaccination campaign for children,” he said.

Israel did not immediately comment on Lazzarini’s remarks. Israeli authorities have previously said they facilitate food deliveries to Gaza despite challenging conditions.

Despite a year of relentless Israeli attacks on Gaza, and intermittent declarations by the IDF and other officials claiming to have defeated Hamas, Israeli tanks and infantry attacked northern Gaza for a third time in force earlier this week, claiming the action was necessary to prevent Hamas “regrouping”.

Along with evacuation orders, the IDF has ordered the closure once again of several hospitals in northern Gaza, including the Kamal Adwan, Indonesia and al-Awda hospitals. The Al Mezan Center for Human Rights described the situation as “deja vu” on social media, adding: “We all know the horrors that follow such orders.”

Among those raising the alarm has been the international medical group Médecins Sans Frontières, whose staff described the situation in northern Gaza.

“All of a sudden, I was told that we had to move from the north,” said Mahmoud, an MSF guard, who left Jabaliya at night to find refuge at the MSF guest house in Gaza City.

“We left our home in despair, under bombs, missiles and artillery. It was very, very difficult. I would prefer to die than to be displaced to the south; my home is here, and I do not want to leave.”

Sarah Vuylsteke, an MSF project coordinator in Gaza, said: “The latest move to forcefully and violently push thousands of people from northern Gaza to the south is turning the north into a lifeless desert while aggravating the situation in the south.

“Access to water, healthcare and safety is already almost nonexistent, and the thought of more people fitting into this space is impossible to imagine,” she said. “People have been subjected to endless displacement and relentless bombing for the past 12 months. Enough is enough. This must stop now.”

The UN’s Office for the Coordination of Humanitarian Affairs (OCHA) said hundreds of thousands of Palestinians and dozens of humanitarian facilities had been affected by the Israeli military’s latest forced evacuations across north, central and southern Gaza.

Between Saturday and Monday, evacuations orders were in place for several areas in the north of the Palestinian territory as well as areas in central Deir al-Balah and southern Khan Younis.

“There are growing risks that humanitarian access will be further constrained, particularly between southern and northern Gaza,” OCHA warned.

Related: Israeli military deploys fourth division in Lebanon ground offensive

At least 18 people were killed in the latest Israeli military strikes on Gaza overnight, Palestinian medics said on Wednesday, including five children and two women. Two strikes hit tents for displaced people in the urban Nuseirat and Bureij refugee camps in central Gaza.

The bodies of nine people, including three children, were brought to the al-Aqsa hospital in nearby Deir al-Balah. An Associated Press journalist saw the bodies at the morgue.

In northern Gaza, an Israeli strike hit a family home in the Jabaliya refugee camp, killing at least nine people, according to the Civil Defence, a rescue agency operating under the Hamas-run government.

The dead were taken to the al-Ahli hospital, which said two women and two children were among those killed. Footage shared by the Civil Defence showed first responders recovering dead bodies and body parts from under the rubble.

The latest deaths brought the number of Palestinians killed in Gaza by Israeli operations since October 2023 to 42,010 with a further 97,720 injured, according to Gaza’s ministry of health.