The army released a video it said showed a tunnel entrance in an outdoor area of Al Shifa, Gaza's biggest hospital.
The video, which Reuters could not immediately verify, showed a deep hole in the ground, littered with and surrounded by concrete and wood rubble and sand.
It appeared the area had been excavated; a bulldozer appeared in the background.
The army said its troops also found a vehicle in the hospital containing a large number of weapons.
Hamas said in a statement late on Thursday that claims by the Pentagon and US State Department that the group uses Al Shifa for military purposes "is a repetition of a blatantly false narrative, demonstrated by the weak and ridiculous performances of the occupation army spokesman".
The United States was confident in an assessment from its own intelligence agencies on Hamas activities in Al Shifa hospital and would neither share nor elaborate on it, White House spokesperson John Kirby said on Thursday.
Israeli forces have raided Al Shifa hospital, where it claims Hamas has a command centre.
The two telecoms companies in Gaza said all services in the territory were down as energy supplies had run out.
Israel refuses fuel imports, saying Hamas could use them for military purposes.
With communications out and in the absence of fuel, the UN agency for Palestinian refugees, UNRWA, said it was impossible to co-ordinate humanitarian aid truck convoys.
"If the fuel does not come in, people will start to die because of the lack of fuel. Exactly as from when, I don't know. But it will be sooner rather than later," UNRWA commissioner-general Philippe Lazzarini said.
Palestinian civilians have borne the brunt of Israel's weeks-long military campaign in retaliation for an October 7 attack by Hamas that Israel says killed 1200 people, mostly civilians.
Gaza health authorities deemed reliable by the United Nations say at least 11,500 people have been confirmed killed in an Israeli bombardment and ground invasion - more than 4700 of them children.
Israeli Prime Minister Benjamin Netanyahu was asked by US television's CBS News on Thursday whether Israel's killing of thousands of Palestinians would fuel hatred in a new generation.
Palestinian civilians have borne the brunt of Israel's weeks-long military campaign.
Netanyahu said Israel's attempts to minimise civilian casualties were "not successful", and he accused Hamas of preventing civilians moving to safer locations.
"Any civilian death is a tragedy. And we shouldn't have any because we're doing everything we can to get the civilians out of harm's way, while Hamas is doing everything to keep them in harm's way," Netanyahu said.
"So we send leaflets, (we) call them on their cell phones, and we say: 'leave'. And many have left," he said.
The Israeli military's chief of staff said Israel was close to destroying Hamas' military system in the northern Gaza Strip and there were signs the army was taking its campaign to other parts of the enclave of 2.3 million people.
Israel distributed pamphlets telling civilians to leave four towns in southern Gaza, areas Gazans had been previously told would be safe.
Israeli officials said Hamas held some of the 240 hostages taken by gunmen on October 7 in the hospital complex.
The body of a woman hostage was recovered by troops in a building near Al Shifa on Thursday, the army said.
The Israeli military says it's close to destroying Hamas' military system in northern Gaza.
Military equipment including Kalashnikov rifles and rocket-propelled grenades were also found in the building, it said.
Human Rights Watch said hospitals had special protections under international humanitarian law.
EU foreign policy chief Josep Borrell, on his first visit to Israel since the October 7 Hamas attack, called on Israel to do more to protect civilians in Gaza.
"I understand your rage but let me ask you not to be consumed by rage," Borrell said.
Israeli Foreign Minister Eli Cohen said Hamas was to blame not only for the October 7 attack but also for the current plight of Palestinians in Gaza.
