UN chief António Guterres said on Sunday he was "shocked by harrowing levels of death, injury and destruction" in north Gaza, where Israeli forces are carrying out attacks they say aim to prevent Hamas regrouping.
"The plight of Palestinian civilians trapped in north Gaza is unbearable," Stéphane Dujarric, Guterres' spokesperson, said.
"The secretary-general is shocked by the harrowing levels of death, injury and destruction in the north, with civilians trapped under rubble, the sick and wounded going without life-saving health care, and families lacking food and shelter."
Israel, vowing to stop Hamas militants from regrouping in the north of the Palestinian territory, launched a major air and ground assault on 6 October this year.
The UN spokesperson said that according to Gaza's health ministry, hundreds of people have been killed in recent weeks, and more than 60,000 others were forced to flee.
"Repeated efforts to deliver humanitarian supplies essential to survive — food, medicine and shelter — continue to be denied by the Israeli authorities, with few exceptions, putting countless lives in peril," Dujarric said.
"In the name of humanity, the secretary-general reiterates his calls for an immediate ceasefire, the immediate and unconditional release of all hostages, and accountability for crimes under international law."
Later on Sunday, Gaza civil defence agency spokesperson Mahmoud Bassal criticised Israel's ongoing "siege" in the areas of Jabalia, Beit Hanoun and Beit Lahia in northern Gaza, saying 100,000 people were trapped.
"For 22 days, not a drop of water or bread has entered the northern Gaza Strip," Bassal said in a statement.
"The occupation forces kill anyone who tries to provide services to the residents of north Gaza."
Beit Lahia resident Bilal al-Hajri, 25, said the siege was unleashing a "famine" in the area.
"We are really dying under a tight siege and famine," he told AFP.
"None of us can leave our homes to even provide some food and drink ... anyone who leaves is targeted."
The Israeli military has come under strong criticism for its operation in north Gaza, where tens of thousands of civilians are trapped.
The military says the goal of the assault is to destroy the operational capabilities it says Hamas is trying to rebuild in the north.
Since Israel's assault on Gaza, more than 42,800 people have been killed in the enclave, according to the Gaza health ministry.
Hamas' , which triggered the conflict, killed 1,200 people in Israel, with more than 250 taken hostage.
