Gaza City, Gaza - The Israeli attack on Al-Shifa hospital has set off a wave of international condemnation.

The army had announced overnight it was carrying out "a precise and targeted operation against Hamas in a specified area" of the hospital where the UN estimates there are at least 2,300 patients, staff and displaced Palestinians.

Israel has not provided concrete evidence for its claims.

Tanks have surrounded and entered the compound, with as many as 200 people reportedly being held in the hospital yard by Israeli forces, some of whom have been mistreated, according to Al Jazeera.

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Israel has long accused Hamas of using Gaza hospitals as bases, digging tunnels under Al-Shifa and exploiting patients as human shields. In response to the raid, the organization has blamed US President Joe Biden, who it says is "wholly responsible for the assault."

"The adoption by the White House and the Pentagon of the occupation's false claim that the resistance is using the Al-Shifa medical complex for military ends has given the green light to the occupation to commit more massacres against civilians," a statement added.

The Palestinian Authority that runs the occupied West Bank also denounced the raid as a "flagrant violation" of international law, demanding world action to protect patients and civilians inside the facility.

The White House had expressed some concern for the safety of civilians shortly after the raid began.

"We do not support striking a hospital from the air and we don’t want to see a firefight in a hospital where innocent people, helpless people, sick people trying to get medical care they deserve are caught in the crossfire," a National Security Council spokesperson said.

"Hospitals and patients must be protected."

Earlier, the White House had said unspecified US intelligence corroborated Israel's claim that Hamas and another Palestinian militant group, Islamic Jihad, had buried an operational "command and control node" under Al-Shifa.

But with hundreds of patients – including premature babies – in grave danger, the UN's humanitarian chief demanded immediate action to "rein in the carnage".

"As the carnage in Gaza reaches new levels of horror every day, the world continues to watch in shock as hospitals come under fire, premature babies die..."

"An entire population is deprived of the basic means of survival. This cannot be allowed to continue," Martin Griffiths said.

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Griffiths stressed in a separate video statement he understood Israel wanted to find Hamas leaders, but insisted that was no excuse for turning hospitals into a battlefield.

"Hamas must not, should not, use a place like a hospital as a shield for their presence," he said.

The International Committee of the Red Cross also said it was "extremely concerned about the impact on sick and wounded people, medical staff, and civilians," while World Health Organization chief Tedros Adhanom Ghebreyesus said the agency had lost touch again with staff at the hospital.

"We're extremely worried for their and their patients' safety."

Israel's action were "going too far and cannot be accepted", said Norwegian Foreign Minister Espen Barth Eide in an email to AFP.

"It aggravates an already horrible humanitarian situation in Gaza."