The head of the UN agency for Palestinian refugees (Unrwa) says Israel is continuing to prevent humanitarian missions from reaching northern Gaza with critical supplies, including food and medicine.

“Hospitals have been hit and are left without power while injured people are left without care,” Philippe Lazzarini wrote on X.

He also said Unrwa’s remaining shelters were so overcrowded that displaced people were “forced to live in the toilets”, and cited reports that people trying to flee were being killed.

The Israeli military has been intensifying a weeks-long offensive in parts of northern Gaza against what it said were Hamas fighters who had regrouped there. On Monday residents and medics said Israeli forces were besieging hospitals and shelters for displaced people.

The Israeli military said it was facilitating evacuations of civilians and ensuring hospitals remained operational while it continued “operating against terrorists and terrorist infrastructure”.

Medics at the Indonesian Hospital in northern Gaza told Reuters that Israeli troops stormed a school and detained the men before setting the building ablaze.

Palestinian media also reported on Monday that at least 10 people had been killed by Israeli artillery fire that hit a camp for displaced people at a school in Jabalia refugee camp, a densely-populated urban area to the north of Gaza City.

Israel does not allow the BBC and other international media into Gaza to report independently, making it difficult to verify facts on the ground, so we rely on information from video footage and testimonies.

Graphic videos of the aftermath of the Israeli strike posted online by Gaza’s Hamas-run Civil Defence agency and local journalists appeared to show at least four bodies, including a child and a woman, lying on the ground inside a tented camp.

One of the videos was filmed by a paramedic called Nabila as she ran between the dead and wounded.

“Calm down,” she is heard screaming at a badly hurt woman sitting in a pool of blood, “I swear I don’t have anything to stop the bleeding”.

In a passage pockmarked by shrapnel, she comes across a woman sitting with a baby, who says: “My children are gone, look at them.”

The Israeli military said it was checking the reports.

The Israeli military body responsible for managing crossings into Gaza, Cogat, also announced that 41 aid lorries and six fuel tankers had been transferred to the north over the past day, and that a Unicef mission had been able to deliver polio vaccines to the north.

Cogat said there were also 600 lorry loads of aid waiting to be picked up and distributed at various crossings, most of it by UN agencies.

The UN said no aid was allowed into northern Gaza during the first two weeks of October, when the Israeli military began its offensive in and around Jabalia.

The UN’s acting humanitarian chief said a “trickle” of aid was allowed through last week, after the US warned Israel in a letter to urgently boost access within 30 days or risk having some military assistance cut off.

On Monday, the UN Office for the Co-ordination of Humanitarian Affairs (OCHA) said it had been asking Israeli authorities for four days to access to the Falouja area of Jabalia but had been denied.

The OCHA also shared a video showing an appeal for help from a Jabalia resident who said he was one of 32 people buried underneath a building destroyed in an air strike on Friday.

“Eighteen of us got out. Fourteen people remain under rubble, including little kids. They are two, three and four-year-olds, as well as women. They’re under rubble. Alive. They begged for me to rescue them but I couldn’t,” Shamekh al-Dibes said.

Meanwhile, a representative of the International Committee of the Red Cross (ICRC) who recently visited Gaza City said the suffering for the estimated 400,000 people in the north was “unimaginable”.

“Heavy fighting and evacuation orders are tearing communities apart. While some are desperate to leave, many, especially the elderly, sick, and people with disabilities, are unable to leave. Other stay, believing nowhere is safe,” Stephanie Eller said in a video.

“Hospitals are overwhelmed, struggling with too many patients and lack of fuel, electricity, and water supplies,” she added. “People need food, water, medical care and, above all, a respite from the ongoing hostilities.”

Hadeel Obeid, the chief nurse at the Indonesian hospital, also near Jabalia, said its water supply had been cut off and that was no food for the fourth consecutive day. She also said that the hospital needed permission from the Israeli military to operate its generator.

Israel launched a campaign to destroy Hamas in response to the group’s unprecedented attack on southern Israel on 7 October 2023, in which about 1,200 people were killed and 251 others were taken hostage.

More than 42,000 people have been killed in Gaza since then, according to the territory’s Hamas-run health ministry.