(Reuters) - The largest hospital in Gaza has ceased to function and fatalities among patients are rising, the head of the World Health Organization (WHO) said on Sunday, as a fierce Israeli assault continues in the Hamas-controlled strip.

Hospitals in the north of the Palestinian enclave, including the al-Shifa complex, are blockaded by Israeli forces and barely able to care for those inside, with three newborns dead and more at risk from power outages amid intense fighting nearby, according to medical staff.

Israel says it is homing in on Palestinian Hamas militants who launched deadly attacks in southern Israel on Oct. 7, and says the group has command centers under and near the hospitals.

The WHO managed to speak to health professionals at al-Shifa, who described a "dire and perilous" situation with constant gunfire and bombing exacerbating the already critical situation, Director-General Tedros Adhanom Ghebreyesus said.

"Tragically, the number of patient fatalities has increased significantly," he said in a post on X, formerly known as Twitter, adding that al-Shifa was "not functioning as a hospital anymore".

Tedros joined other top United Nations officials in calling for an immediate ceasefire.

"The world cannot stand silent while hospitals, which should be safe havens, are transformed into scenes of death, devastation, and despair," he said.

The president of Indonesia, home to the world's biggest Muslim population, also called for a ceasefire ahead of meeting U.S. President Joe Biden in Washington on Monday.

"A ceasefire must be implemented soon, we also must accelerate and increase the amount of humanitarian aid, and we must begin peace negotiations," President Joko Widodo said in a video recorded after he took part in an Organization of Islamic Cooperation (OIC) in Riyadh.

He said the world seemed "helpless" in the face of the suffering of the Palestinians. The extraordinary joint Islamic-Arab summit also urged the International Criminal Court to investigate "war crimes and crimes against humanity that Israel is committing" in the Palestinian territories.

Israel says it is trying to free the more than 200 hostages taken by Hamas militants on Oct. 7 and says the hospitals should be evacuated.

The European Union condemned Hamas for using "hospitals and civilians as human shields" in Gaza, while also urging Israel to show "maximum restraint" to protect civilians.

"These hostilities are severely impacting hospitals and taking a horrific toll on civilians and medical staff," European Union foreign policy chief Josep Borrell said on Sunday in a statement issued on behalf of the 27-nation bloc.

White House National Security Adviser Jake Sullivan said Hamas was using hospitals and other civilian facilities to house fighters and weapons, which he said was a violation of the laws of war.

"The United States does not want to see firefights in hospitals where innocent people, patients receiving medical care, are caught in the crossfire and we've had active consultations with the Israeli Defense Forces on this," Sullivan told CBS News.

Israel declared war on Hamas more than a month ago after militants rampaged through southern Israel, killing about 1,200 people, most of them civilians, according to Israeli officials.

Palestinian officials said on Friday that 11,078 Gaza residents had been killed in air and artillery strikes since then, around 40% of them children.

The Israeli military response has also prompted outrage in several cities across the world, where hundreds of thousands of people held ptotests demanding a ceasefire.