Israeli and Palestinian officials said military operations were taking place at Gaza City's Al-Shifa hospital -- the focal point of days of deadly fighting and nearby aerial bombardments.
Youssef Abu Rish, an official from the health ministry in Gaza who is inside the hospital, told AFP he could see tanks inside the complex and "dozens of soldiers and commandos inside the emergency and reception buildings".
The Israeli occupation army described it as "a precise and targeted operation against Hamas in a specified area" of the facility.
After sharp warnings from the United States and others that Al-Shifa must be protected, Israel said the raid was being executed based on "an operational necessity".
The United Nations has said it estimates that at least 2,300 people -- patients, staff and displaced civilians -- are inside and may be unable to escape because of fierce fighting.
Witnesses have described conditions inside the hospital as horrific, with medical procedures taking place without anaesthetic, families with scant food or water living in corridors, and the stench of decomposing corpses filling the air.
"There are bodies littered in the hospital complex and there is no longer electricity at the morgues," hospital director Mohammad Abu Salmiya said prior to the operation.
Anticipating a fierce backlash against the hospital raid, the Israeli occupation military said it had provided evacuation routes for civilians and given authorities in Hamas-run Gaza 12 hours' notice that any military operation inside must cease.
"Unfortunately, it did not," the Israeli military said, again calling on "all Hamas terrorists present in the hospital to surrender".
The Israeli army claimed its ground teams included medics and Arabic speakers "who have undergone specified training to prepare for this complex and sensitive environment".
The intent was that "no harm is caused to the civilians being used by Hamas as human shields", it added.
Prime Minister Benjamin Netanyahu has vowed to destroy Hamas in response to its attacks on October 7, which killed an estimated 1,200 people and saw 240 captives taken to Gaza.
The Hamas-run health ministry in Gaza says Israel's ensuing aerial bombardment and ground offensive have killed 11,320 people, mostly civilians, including thousands of children.
White House warnings
The ministry's Abu Rish called on "the international community and the United Nations to intervene immediately and urgently to stop the Israeli storming operation".
He said that among the thousands inside the hospital were "650 ailing people and thousands of injured people".
A journalist inside the hospital who is collaborating with AFP said Israeli soldiers were interrogating people on Wednesday morning, among them patients and doctors.
The White House reiterated its concerns 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," a National Security Council spokesperson said.
The official added that there should not be a situation in which "innocent people, helpless people, sick people trying to get medical care they deserve are caught in the crossfire".
Earlier, the White House had said that US intelligence sources corroborated Israel's claim that Hamas and another Palestinian militant group, Islamic Jihad, had buried an operational "command and control node" under Al-Shifa.
Hamas, which has repeatedly denied the claims, on Wednesday said US President Joe Biden was "wholly responsible" for the assault, accusing his administration of giving Israel "the green light... to commit more massacres against civilians".
Israel has said that the military use of the hospital "jeopardises" its "protected status under international law", a claim that many international human rights lawyers refute.
Citing the Palestinian health ministry, UN humanitarian agency OCHA said 40 patients had died in Al-Shifa on Tuesday, while hospital director Abu Salmiya said 179 bodies had been interred in a mass grave inside the complex.
'I was bleeding'
The situation in Gaza's other hospitals is also dire, with the UN saying 22 of 36 are not functional due to lack of generator fuel, damage and combat.
"The 14 hospitals remaining open have barely enough supplies to sustain critical and life-saving surgeries and provide inpatient care, including intensive care," the WHO said.
The WHO "has warned that the evacuation of hospitals in the north, as demanded by the Israeli military, would be a 'death sentence' for some patients, because operational hospitals in the south cannot admit more patients", according to an update from OCHA.
The humanitarian crisis also includes 1.5 million people who, according to the UN, have fled southwards after Israel told them to leave the northern half of the territory.
Even escaping the fighting is dangerous. Wounded Palestinians told AFP how they were hit by a strike on their way south.
"I walked around three to four kilometres (around two miles) while I was bleeding," said Hasan Baker, whose head and left hand were bandaged. "There was no possibility for any ambulance to enter the area."
Hostage talks
Israeli leaders have so far rejected any calls for a ceasefire in the five-week-old war until hostages are released.
Abu Obeida, spokesman for Hamas's military wing, said Monday that Israel had asked for the release of 100 hostages, while the militants want 200 Palestinian children and 75 women freed from Israeli prisons.
"We informed the mediators we could release the hostages if we obtained five days of truce... and passage of aid to all of our people throughout the Gaza Strip, but the enemy is procrastinating," Abu Obeida said in an audio statement.
Qatari foreign ministry spokesman Majed bin Mohammed Al-Ansari, who is helping oversee talks on a hostage deal, said the "deteriorating" situation in Gaza was hampering efforts to find agreement.
With pressure building on the Israeli government, Prime Minister Netanyahu said he was "working relentlessly" to get the hostages out.
Relatives of the hostages set out Tuesday on a five-day protest march from Tel Aviv to the prime minister's office in Jerusalem to call for the captives' release, the Hostages and Missing Families Forum said.
The group later demanded the government "approve a deal tonight to bring home all hostages from Gaza".
US President Biden voiced confidence that a deal could still happen, telling the families of the hostages: "Hang in there. We're coming."
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