The Israeli Defense Forces (IDF) said it launched a "precise operation" at al-Shifa Hospital in Gaza on Monday morning.
"The operation is based on intelligence information indicating the use of the hospital by senior Hamas terrorists," it added.
Israel has repeatedly accused Hamas, which is classified as a terror group by the US, the EU, Germany and others, of running its operations from — or under — hospitals and other civilian facilities.
Witnesses in Gaza City told the AFP news agency that the al-Rimal neighborhood, where the Shifa Hospital is located, had been hit by airstrikes and that tanks had surrounded the hospital complex itself.
Gaza's Hamas-run Health Ministry accused Israel of using "fabricated narratives" to "justify the storming" of the hospital, which it called a violation of international humanitarian law.
"We hold the Israeli occupation responsible for the lives of medical staff, patients and displaced persons inside al-Shifa Medical Complex," the ministry said in a Facebook post.
"The Israeli military attack aims to continue destroying the health system in northern Gaza," it added.
In its statement on Monday, the IDF said its troops had been "instructed on the importance of operating cautiously, as well as on the measures to be taken to avoid harming the patients, civilians, medical staff, and medical equipment," before the operation started.
The Israeli army previously entered the hospital in a military operation at the end of last year, uncovering what it said were tunnels used by Hamas.
Israel to send delegation to Qatar talks
A delegation from Israel is set to depart for Qatar on Monday to take part in cease-fire talks, Israeli news website Ynet reported.
The negotiators reportedly include Mossad chief David Barnea.
The Associated Press (AP) news agency reported that the team was authorized to attend the talks after a meeting of Israel's Security Cabinet and War Cabinet on Sunday, which outlined a mandate for the negotiators.
At the talks in Doha, mediators from Qatar, Egypt and the US will attempt to make progress on a temporary cease-fire, as well as a deal to free Israeli hostages in Gaza and release Palestinian prisoners in Israel.
Oxfam accuses Israel of 'deliberately' blocking aid to Gaza
Global NGO Oxfam has accused Israel of "systematically and deliberately" blocking the entry of humanitarian aid into Gaza.
In a report published on Monday, the organization said that "unjustifiably inefficient" inspection rules were causing aid trucks to be stuck in 20-day-long queues on average.
It also cited the rejection of "dual-use" items — civilian goods that also have potential military uses like backup generators and torches. In one Oxfam shipment, the group said water bags and water testing kits were initially rejected with no reason provided before later being permitted entry into Gaza.
Oxfam also denounced "attacks on aid workers, humanitarian facilities and aid convoys" and "access restrictions" for relief staff.
By putting in place these barriers, Oxfam alleged that Israel was defying an order by the International Court of Justice to boost aid deliveries to Gaza, while also failing its legal responsibility to protect people in land it occupies.
"The ICJ order should have shocked Israeli leaders to change course, but since then conditions in Gaza have actually worsened," said Oxfam Middle East and North Africa director Sally Abi Khalil.
"Israeli authorities are not only failing to facilitate the international aid effort but are actively hindering it."
Israel has previously defended its policies and has argued that the UN should send more aid to Gaza in order to meet the need that is there.
