The Israeli military said it was carrying out a raid on Wednesday against Hamas militants in Al Shifa Hospital, having urged them to surrender with thousands of Palestinian civilians still sheltering inside Gaza Strip’s biggest hospital.

Dr. Munir al-Bursh, director-general of the Gaza health ministry, told Al Jazeera television that Israeli forces had raided the western side of the medical complex.

“There are big explosions and dust entered the areas where we are. We believe an explosion occurred inside the hospital,” Bursh said.

Hours later, Gaza health ministry spokesperson Ashraf al-Qidra told Al Jazeera: “The occupation army is now in the basement, and searching the basement. They are inside the complex, shooting and carrying out bombings”.

Israeli forces first raided the surgery and emergency departments, Mohammed Zaqout, the Gaza health ministry’s director of hospitals, told Al Jazeera.

Reuters was unable to independently confirm the situation at Al Shifa.

Global calls for a humanitarian ceasefire have mounted in recent days, and the fate of Al Shifa has become a focus of international alarm because of worsening conditions in the facility, where thousands of patients, medical staff and displaced people have been trapped during the Israeli assault on Gaza in the past five weeks.

Israel has said that Hamas has a command centre underneath Al Shifa and uses the hospital and tunnels beneath it to conceal military operations and to hold hostages. Hamas denies it.

In a statement, the Israel Defence Forces (IDF) said: “Based on intelligence information and an operational necessity, IDF forces are carrying out a precise and targeted operation against Hamas in a specified area in the Shifa hospital.”

The military added: “The IDF forces include medical teams and Arabic speakers, who have undergone specified training to prepare for this complex and sensitive environment, with the intent that no harm is caused to the civilians.”

Israeli army spokesman Lieutenant Colonel Peter Lerner told CNN the hospital and compound were for Hamas “a central hub of their operations, perhaps even the beating heart and maybe even a centre of gravity.”

The U.S. said on Tuesday that its own intelligence supported Israel’s conclusions.

Hamas said on Wednesday that U.S. announcement had effectively given a “green light” for Israel to raid the hospital. The group said it held Israel and U.S. President Joe Biden fully responsible for the operation.

“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. Hospitals and patients must be protected,” a White House National Security Council spokesperson said in a statement.

Israeli forces have waged fierce street battles against Hamas fighters over the past 10 days before advancing into the centre of Gaza City and surrounding Al Shifa.

Israel has sworn to destroy Hamas in retaliation for the militants’ cross-border assault into Israel on Oct 7. Israel says Hamas killed 1,200 people in the rampage and took more than 240 hostage.

In the West Bank, a separate Palestinian enclave not controlled by Hamas, Palestinian Authority Health Minister Mai Alkaila said Israel was “committing a new crime against humanity, medical staff and patients by besieging” Al Shifa.

“We hold the occupation forces fully responsible for the lives of the medical staff, patients and displaced people in Al Shifa,” Alkaila said in a statement.

Al Shifa is a sprawling complex of buildings and courtyards a few hundred metres from Gaza City’s fishing port. Buildings on the western side of the complex, which the Gaza official said was the site of the raid, include the internal medicine and dialysis departments.

Hamas says 650 patients and 5,000 to 7,000 other civilians are trapped inside the hospital grounds, under constant fire from Israeli snipers and drones. Amid shortages of fuel, water and supplies, it says 40 patients have died in recent days.

Thirty-six babies are left from the neo-natal ward after three died. Without fuel for generators to power incubators, the babies were being kept as warm as possible, lined up eight to a bed.

Palestinians trapped in the hospital dug a mass grave on Tuesday to bury patients who died and no plan was in place to evacuate babies despite Israel announcing an offer to send portable incubators, Qidra, Gaza’s health ministry spokesman, said.

Qidra said there were about 100 bodies decomposing inside and no way to get them out.

U.N. Secretary-General Antonio Guterres was deeply disturbed by the “dramatic loss of life” in the hospitals, his spokesman said. “In the name of humanity, the secretary-general calls for an immediate humanitarian ceasefire,” the spokesman told reporters.

Medical officials in Hamas-run Gaza say more than 11,000 people are confirmed dead from Israeli strikes, around 40% of them children, and countless others were trapped under rubble.

Around two-thirds of Gaza’s 2.3 million people have been made homeless, unable to escape the territory where food, fuel, fresh water and medical supplies are running out.