AMNA NAWAZ: Israel says, underneath Gaza's main hospital, Al Shifa, is a Hamas military command center.

Today, President Biden warned Israel to be -- quote - - "less intrusive" in its operations there.

Nick Schifrin reports on the legal arguments over whether the hospital and what's underneath it are legitimate military targets, even as hundreds of patients and medical staff remain inside facing dire conditions.

NICK SCHIFRIN: They were already the most vulnerable, and now they are helpless, babies in Gaza's Al Shifa Hospital born so prematurely they needed incubators to survive.

But, today, they're warmed only by each other after the generators that powered the incubators ran out of fuel.

Four were already orphans delivered by a Caesarean section after their mothers had died, said Dr. Medhat Abbas, a spokesman for the Hamas-run Gaza Health Ministry.

DR. MEDHAT ABBAS, Gaza Health Ministry: This aluminum foil is just kept around the babies to protect them from the cold weather.

Without having proper temperature for them, they immediately die.

I hope, I hope that they will remain alive, despite the disaster in which this hospital is passing through.

NICK SCHIFRIN: Death is on the mind of everyone inside Shifa these days, where the hallways are lined with the injured.

The doctors have no food or water.

And the WHO said today Shifa was no longer functioning as a hospital.

FRANCOISE BOUCHET-SAULNIER, Doctors Without Borders: There's no space for humanitarian negotiation in this conflict, which is unprecedented for us.

NICK SCHIFRIN: Francoise Bouchet-Saulnier is the senior legal adviser for international humanitarian law at Doctors Without Borders, which she joined 35 years ago.

FRANCOISE BOUCHET-SAULNIER: There's no drugs, no anesthesia, nothing.

There's no water.

So this, imposing a siege, a complete siege on humanitarian relief is a violation of humanitarian law.

COL. PNINA SHARVIT BARUCH (RET.

), Israeli Defense Forces: Hamas is using this fuel to fight us.

It's only logical that, if there is no way to make sure that this fuel will get to the hospital, Israel will be supplying fuel to its enemies to fight against it.

It won't get to the hospitals anyway.

And that I don't think -- the law doesn't say that you have to do that.

The law has a logic.

NICK SCHIFRIN: Retired Colonel Pnina Sharvit Baruch was a legal adviser to the Israeli Defense Forces and is now with the Institute for National Security Studies.

For Israel, Shifa is not just a hospital.

Israeli, U.S. intelligence and reporters who have been called to meetings by Hamas officials in the hospital say it is a Hamas headquarters.

REAR ADM. DANIEL HAGARI, Spokesperson, Israeli Defense Forces (through translator): Hamas has turned hospitals into command-and-control centers.

NICK SCHIFRIN: Last month, top Israeli military spokesman Admiral Daniel Hagari showed what he called an illustration of Hamas' tunnels under Shifa Hospital.

REAR ADM. DANIEL HAGARI: Hamas uses Shifa Hospital as a shield for Hamas terror infrastructure.

Hamas wages war from hospitals.

NICK SCHIFRIN: That's an allegation that Hamas quickly denied.

SALAMA MAROUF, Hamas Government Media Office: The Israelis have failed to provide a single piece of evidence to prove that this facility has tunnels underneath or any command center underneath.

NICK SCHIFRIN: And no matter what's underneath, international humanitarian law protects the hospital's doctors and patients, says Bouchet-Saulnier.

FRANCOISE BOUCHET-SAULNIER: The hospital cannot lose this protection if some patients and doctors remains inside.

I mean that the parties are not relieved of their duty to take all precautions to ensure that the military advantage they will obtain through pursuing their attack will not affect the patient, the doctors and the civilian in a way that is disproportionate with the military advantage.

REAR ADM. DANIEL HAGARI: We're now entering into the area of the hospital where we had found the evidence.

NICK SCHIFRIN: But Israel argues the military advantage is vital and Hamas hides its military assets in multiple hospitals.

Today, Hagari visited the Rantisi Hospital and showed what he called evidence of Hamas fighters, motorcycles they used on October 7 and where they kept hostages.

REAR ADM. DANIEL HAGARI: A woman, clothes and a rope, a rope next to the legs.

And look above this.

Look above it.

It's a baby bottle.

COL. PNINA SHARVIT BARUCH: Well, hospitals have special protected status under international humanitarian law, but they can lose the status when they are actually used by the enemy forces.

You still need to give an advance warning telling the enemy to stop the military use before attacking the hospital.

But if the enemy continues to do so, then a hospital can also become a lawful military target.

NICK SCHIFRIN: An active-duty Israeli military legal adviser told "PBS NewsHour" today it had provided -- quote -- "more than ample warning," including by speaking directly to hospital administrators, opening corridors so civilians who had been living on the Shifa compound could leave, and delaying its ground operation.

But it's not only questions of law.

Israel must also take into account international political pressure, including from its most important ally today.

JOE BIDEN, President of the United States: It's my hope and expectation that there will be less intrusive action relative to the hospital.

NICK SCHIFRIN: Tonight, Israel surrounds the hospital.

As it decides what to do next, there are still thousands of Gazans living on the Shifa compound who could be caught in the crossfire.

For the "PBS NewsHour," I'm Nick Schifrin.