The statement comes after Israel sent tanks, troops and armoured bulldozers into Gaza in a "targeted raid" overnight that the military said destroyed multiple sites before withdrawing.
"(Ezzedine) Al-Qassam Brigades estimates that the number of Zionist prisoners who were killed in the Gaza Strip as a result of Zionist strikes and massacres has reached almost 50," the group said in a statement on its Telegram channel.
Israel has been carrying out relentless strikes on the Gaza Strip which the Palestinian health ministry said have killed more than 7,000 people, mainly civilians -- a toll expected to rise substantially if Israeli troops massed near the border thrust across.
On the 20th day of Israel's bloodiest war on Gaza yet, the UN humanitarian coordinator for the Palestinian territories, Lynne Hastings, said that despite the Israeli military issuing warnings to people in Gaza City to leave, "advance warnings make no difference".
She said in a statement that when evacuation routes are bombed, "people are left with nothing but impossible choices. Nowhere is safe in Gaza."
Black smoke billowed into the sky after a blast in the grainy night-vision footage the Israeli military released hours after Prime Minister Benjamin Netanyahu declared preparations for a ground war were under way.
The operation in northern Gaza came in "preparation for the next stages of combat", the military said.
The black-and-white video showed armoured vehicles moving near Gaza's border fence. Other footage appeared to show an air strike and buildings being targeted.
The raid came after Netanyahu delivered a nationally televised address to Israelis still furious after the October 7 attacks, telling them "we are in the midst of a campaign for our existence".
International alarm has increased amid growing shock about the scale of human suffering inside the besieged Palestinian territory where Israel has cut off most water, food, fuel and other basic supplies.
In southern Gaza, a grieving Umm Omar al-Khaldi told AFP how she saw her neighbours being killed in an Israeli strike that reduced the house to rubble, with many feared buried beneath.
"We saw them getting bombarded -- the children got bombarded while their mother was hugging them," the woman said, desperately pleading for help from the outside world.
"Where are the Arabs, where is humanity?" she asked.
Amnesty International in a statement called for an immediate ceasefire to ensure "access to life-saving aid for people in Gaza amidst an unprecedented humanitarian catastrophe".
The human rights group's chief Agnes Callamard said: "Serious violations of international humanitarian law, including war crimes, by all parties to the conflict continue unabated."
Surging death toll
The war's surging death toll is by far the highest since Israel unilaterally withdrew from the small coastal territory in 2005 -- a period that has seen four previous Gaza wars.
Entire neighbourhoods have been razed, surgeons are operating without anaesthetic, and ice-cream trucks have become makeshift morgues.
In chaotic scenes, volunteers and neighbours have clawed, sometimes with their bare hands, through shattered concrete and sand to pull out civilian casualties.
All too often they recover only corpses which have piled up, wrapped in bloodstained white shrouds.
In Brussels on Thursday, European Union leaders debated calling for "pauses" in the war so aid can get in.
The 27-nation bloc has long been split between more pro-Palestinian members such as Ireland and Spain, and staunch backers of Israel including Germany and Austria.
"What we want is the killing and the violence to stop so that humanitarian aid can get into Gaza, where innocent Palestinian people are suffering, and also to allow us to get EU citizens out," Irish Prime Minister Leo Varadkar said.
US President Joe Biden, a strong supporter of Israel, earlier joined the calls for it to "protect innocent civilians" and to follow the "laws of war" as it pursues Hamas.
French President Emmanuel Macron warned Wednesday that "a massive intervention that would put civilian lives at risk would be an error".
And Jordan's King Abdullah II said anger at the suffering could "lead to an explosion" in the Middle East.
'Raining down hellfire'
Netanyahu -- amid the growing calls to temper the ferocious bombing campaign -- said Israel had been "raining down hellfire on Hamas" and killing "thousands of terrorists".
He said his war cabinet and the military would determine the timing of a "ground offensive" with the goal to "eliminate Hamas" and "bring our captives home".
But he stressed that "I will not detail when, how or how many" forces would take part.
Netanyahu also acknowledged for the first time that he would have to explain the security lapses exposed on October 7.
"The fault will be examined and everyone will have to give answers, including me," he said. "But all this will happen later."
*This story was edited by Ahram Online
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