Israeli military strikes were reported to have killed at least 40 Palestinians across the Gaza Strip as Israeli forces tightened their squeeze around Jabalia in the north of the enclave on Tuesday, amid fierce battles with Hamas-led fighters.

Palestinian health officials said at least 11 people were killed by Israeli fire near Al-Falouja in Jabalia, the largest of Gaza's eight historic refugee camps, while 10 others were killed in Bani Suhaila in eastern Khan Younis in the south when an Israeli missile struck a house.

Earlier on Tuesday, an Israeli airstrike destroyed three houses in the Sabra suburb of Gaza City, and the local civil emergency service said they recovered two bodies from the site, while the search continued for 12 other people who were believed to have been in the houses at the time of the strike.

Five others were killed when a house was struck in the Nuseirat camp in central Gaza.

Jabalia has been the focus of an Israeli offensive for more than 10 days, with troops returning to areas of the north that came under heavy bombardment in the early months of the year-long war.

Israeli troops appear to be 'cutting off' north Gaza: UN

The operation has raised concerns among Palestinians and UN agencies that Israel wants to clear residents from the north of the crowded enclave, a charge it has denied.

The United Nations human rights office said on Tuesday the Israeli military appeared to be "cutting off north Gaza completely from the rest of the Gaza Strip."

"Amid intense ongoing hostilities and evacuation orders in northern Gaza, families are facing unimaginable fear, loss of loved ones, confusion, and exhaustion. People must be able to flee safely, without facing further danger," Adrian Zimmerman, ICRC (International Committee of the Red Cross) Gaza head of sub-delegation, said in a statement.

"Many, including the sick and disabled, cannot leave, and they remain protected under international humanitarian law — all possible precautions must be taken to ensure they remain unharmed. Every person displaced has the right to return home in safety," he said.

WATCH | Israeli airstrikes kill multiple civilians, including at UN-run school distributing polio vaccines: 

Israel criticized for attacks on Gaza civilians as it strikes northern Lebanon

Israel is facing renewed criticism after airstrikes in Gaza killed multiple civilians, including people at a UN-run school distributing polio vaccines. Meanwhile, there is anger in Lebanon after a deadly air attack in the north. Israel vowed to continue striking at Hezbollah throughout the country.

The Israeli military has now encircled the Jabalia camp and sent tanks into nearby Beit Lahiya and Beit Hanoun towns, with the declared aim of stamping out Hamas fighters who are trying to regroup there.

The Israeli military has told residents to leave their homes and head to safety in southern Gaza. Palestinian and UN officials say there was no place safe in Gaza.

Around 400,000 people remain in north, UN estimates

Israeli officials said evacuation orders were aimed at separating Hamas fighters from civilians and denied that there was any systematic plan to clear civilians out of Jabalia or other northern areas.

Hamas's armed wing said fighters were engaged in fierce battles with Israeli forces in and around Jabalia.

Zimmerman also urged for health facilities in the north to be protected, saying hospitals there were struggling to provide medical services.

Gaza's health ministry said the army ordered the three hospitals operating there to evacuate, but medical staffers said they were determined to continue their services even though they are overwhelmed by the growing number of casualties.

On Monday, UN Secretary General António Guterres condemned the level of civilian casualties in northern Gaza.

The northern part of Gaza is home to well over half the territory's 2.3 million people and hundreds of thousands of residents were forced to flee their homes amidst heavy bombing in the first phase of Israel's assault on the territory.

Around 400,000 people remained, according to United Nations estimates.

Israel launched the offensive against Hamas after its Oct. 7, 2023, attack on Israel, in which 1,200 people were killed and around 250 taken hostage to Gaza, according to Israeli tallies. More than 42,000 Palestinians have been reported killed in the offensive so far, according to the Gaza Health Ministry. Most of Gaza's 2.3 million people have been displaced and much of the enclave has been laid to waste.

Polio vaccination campaign underway

Meanwhile, the World Health Organization said on Tuesday it had been able to start its polio campaign in central Gaza and vaccinate tens of thousands of children despite Israeli strikes in the designated protected zone hours before.

As part of an agreement between the Israeli military and Palestinian militant group Hamas, humanitarian pauses in the year-long Gaza war had been due to begin early on Monday to reach hundreds of thousands of children.

However, hours before then, the UN humanitarian office said Israeli forces struck tents near Al-Aqsa Hospital, where it said four people were burned to death.

WATCH | Palestinian children in Gaza get 2nd dose of polio vaccine Monday: 

2nd round of polio vaccinations underway in central Gaza

Hundreds of Palestinian children in central Gaza were immunized with a second dose of the polio vaccine at the UNRWA Deir al-Balah health centre.

The UN Palestinian refugee agency UNRWA said one of its schools in the central Gazan city of Nuseirat, intended as a vaccination site, was hit overnight between Sunday and Monday, killing up to 22 people.

WHO spokesperson Tarik Jasarevic told a Geneva news briefing that over 92,000 children, or around half of the children targeted for polio vaccines in the central area, had been inoculated on Monday.

"What we have received from colleagues is that the vaccination went without a major issue yesterday, and we hope It will continue the same way," he said.

Other humanitarian agencies have previously voiced concerns about the viability of the polio campaign in northern Gaza, where an Israeli offensive is under way.

Aid groups carried out an initial round of vaccinations last month, after a baby was partially paralyzed by the Type 2 polio virus in August, in the first such case in the territory in 25 years.