Israeli tanks have shelled a hospital in the central Gaza Strip, killing at least five people and wounding at least another 50.

Al Jazeera's Stephanie Dekker, reporting from Gaza, said according to early reports the third floor of the al-Aqsa Hospital in Deir al-Balah had been hit by at least three tank shells on Monday.

There was no let-up in the Israeli offensive, with another 31 Gazans killed in a series of strikes on Monday even as the US and the UN demanded an "immediate ceasefire".

And Israel said troops killed 10 Hamas fighters after they came over the border through a network of tunnels that the army says it has been trying to destroy in an intensive four-day ground operation.

On Sunday, at least 72 people were killed in central Gaza during an Israeli assault that destroyed much of the Shujayea district and left charred bodies lying in the streets.

Medics pulled another 68 bodies from the rubble early on Monday, Ashraf al-Qudra, Gaza's emergency services spokesman, said.

Elsewhere, hundreds of people could be seen moving out of the northern Gaza town of Beit Hanoun.

Hospital doctor's account

Al-Aqsa Hospital is the third to be hit by Israeli tank fire since Israel launched its ground offensive in Gaza four days ago.

Al Jazeera's Dekker said the scene of the hospital shelling was an area "where a lot of people had been fleeing to".

"There's extremely heavy shelling going on in that neighbourhood. This is an area you'd think would be relatively safe," she said.

"It's a public hospital; it is run by the government. It was established here before Hamas took over."

A distressed doctor at the scene of the attack told Al Jazeera: "We cannot do anything here for our patients, all the patients, all the injured in the emergency department."

There were no known indications that the hospital was used by armed factions of Hamas.

Mark Regev, spokesman for Israeli Prime Minister Benjamin Netanyahu, told Al Jazeera: "You are allowed to hit targets where their [Hamas] war machine is using to hide rockets."

"I have no doubt Hamas uses, has used and continues to use, hospitals … we do not target civilians," Regev said.

"But I am not aware of this specific situation."

Later on Monday, the Israeli military said an initial investigation suggested that anti-tank missiles were stored near the hospital and that the cache was successfully targeted.

"Civilian casualties are a tragic inevitability of the brutal and systematic exploitation of homes, hospitals and mosques in Gaza," the army said.

Dr Medhat Abbas, director of al-Aqsa Hospital, said any Israeli claim that Hamas was using the hospital "was a big lie".

"I challenge anyone in the international community to prove that. There are no weapons in our hospital," Abbas said.

We are ready for any inspection. Any international organisation can come and make sure we have nothing in our hospital."

"They are professional liars and killers."

Dr Abbas said the hospital was planning to evacuate its remaining patients but had nowhere safe to take them.

"No hospitals are safe in Gaza. If we move them, there is no safe hospital in Gaza," he said.

The hospital's operating theatre and intensive care unit were damaged in the shelling.

Basman Elashi, of El Wafa Hospital, another Gaza hospital that was shelled on Thursday, told Al Jazeera that he could hear very heavy shelling from the area near the hospital.

"We have no place to hide. No place for the children and the women. They are inhumane. They are acting like an animal; they are targeting every home and family in Gaza," Elashi said.

Hamas leader's speech

The ongoing Israeli offensive appeared to only strengthen the resolve of Ismail Haniyah, a senior Hamas leader, to continue fighting.

"Dear blood was spilled today. These massacres, these [killings] against our Palestinian people cannot be overlooked," he said in a televised speech.

"Today's resistance will have the final word, the final strike. Through faith, dignity, the resistance is supreme and the people and their will, will also be supreme.

Separately, Hanan Ashrawi, PLO executive committee member, told Al Jazeera: "We cannot let this situation continue. The people who are paying the price are innocent civilians, women and children."

Against this backdrop of rising civilian deaths, Ban Ki-moon, the UN secretary-general, arrived in the Egyptian capital Cairo for high-level talks on ending the hostilities, with John Kerry, US secretary of state, also expected to fly in later on Monday.

At least 573 Palestinians have been killed since the conflict escalated two weeks ago, most of them civilians.

Eighteen Israeli soldiers, including seven on Monday, and two civilians have died.

Also on Monday, Hamas fired 50 more rockets at Israel, including two at Tel Aviv, causing no injuries or damage, agencies reported.

Since the start of the Israeli operation, Hamas has fired almost 2,000 rockets at Israel.