(AP) — Heavy fighting erupted Monday around a hospital in northern Gaza where thousands of patients and displaced people have been sheltering for weeks, as Israeli forces focus on clearing medical facilities that they say Hamas militants use for cover.

A shell struck the second floor of the Indonesian Hospital, killing at least 12 people, according to the Health Ministry in Hamas-ruled Gaza and a medical worker inside the facility. There was no immediate comment from the Israeli military.

The advance on the Indonesian Hospital came a day after the World Health Organization evacuated 31 premature babies from Shifa Hospital in Gaza City, the territory’s largest, where they were among more than 250 critically ill or wounded patients stranded there days after Israeli forces entered the compound.

The plight of Gaza’s hospitals is at the focus of a battle of narratives over the war’s brutal toll on Palestinian civilians, thousands of whom have been killed or buried in rubble since the six-week-old war was sparked by Hamas’ Oct. 7 rampage into southern Israel.

Israel says Hamas uses civilians as human shields, while critics say Israel’s siege and relentless aerial bombardment amount to collective punishment of the territory’s 2.3 million Palestinians.

Marwan Abdallah, the medical worker at the Indonesian Hospital, said Israeli tanks were operating less than 200 meters (yards) from the hospital, and that Israeli snipers could be seen on the roofs of nearby buildings. As he spoke over the phone, the sound of gunfire could be heard in the background.

Abdallah said the hospital had received dozens of dead and wounded in airstrikes and shelling overnight. He said medical staff and displaced people fear Israel will besiege the hospital and force its evacuation.

Health Ministry spokesman Ashraf al-Qidra, who is now based in southern Gaza, said some 600 patients, 200 healthcare workers, and 2,000 displaced people were sheltering there.

U.N. bodies were able to safely evacuate the babies, who were in critical condition, from Shifa to a hospital in southern Gaza, and plan to transport them to a hospital in neighboring Egypt. Four other babies died in the two days before the evacuation, according to Mohamed Zaqout, the director of Gaza hospitals.

Over 250 patients with severely infected wounds and other urgent conditions remain in Shifa, which could no longer provide most treatment after it ran out of water, medical supplies and fuel for emergency generators amid a territory-wide blackout. Israeli forces battled Palestinian militants outside its gates for days before entering the facility last Wednesday.

Israel’s army said it had strong evidence supporting its claims that Hamas maintained a sprawling command post inside and under the hospital’s 20-acre complex, which includes several buildings, garages and a plaza.

The military released a video showing what it said was a tunnel discovered at the hospital, 55-meter (60-yard) long and about 10 meters (33 feet) below ground. It said the tunnel included a staircase and a firing hole that could be used by gunmen, and ended at a blast-proof door that troops have not yet opened.