Deadly Israeli strikes were also reported in other areas of Gaza on Thursday.
Nasser Hospital in the southern city of Khan Yunis said it had received the bodies of 24 people killed in Israeli military operations there and in nearby Rafah.
The Israeli military recently stepped up its ground offensive in parts of northern Gaza, saying it was preventing Hamas from regrouping there.
The UN said that Beit Lahia, as well as the nearby towns of Beit Hanoun and Jabalia, were under siege and that little humanitarian aid had been delivered since early October.
Pictures from the site of the airstrike in Beit Lahia on Thursday morning showed piles of rubble and twisted metal about 55 meters (185 feet) from Kamal Advan Hospital.
Its director, Dr. Husam Abu Safiya, said “dozens of people are dead or missing.”
“The bodies arrive at the hospital in pieces,” he told AFP. “But there are no ambulances, the health system in northern Gaza is on its knees.”
Dr. Abu Safiya said the hospital was only able to provide first aid to most of the injured because the Israeli forces did not provide enough medical supplies.
The Israeli military said in a statement that it “struck Hamas terrorist infrastructure in the Beit Lahiya area” overnight and that Hamas members were operating there, including some involved in the October 7, 2023 attack on Israel that led to the Gaza War.
“Prior to the strikes, numerous steps were taken to reduce the risk to civilians, including the use of aerial surveillance, warning the population in the area to evacuate the active combat zone, and additional intelligence information,” the statement added. “The incident is under review.”
Israel’s ground offensive in northern Gaza over the past five weeks has displaced up to 130,000 people.
The UN reports that 75,000 people remain under siege with reduced water and food supplies in Beit Lahia, Jabalia and Beit Hanoun.
Last week, a Human Rights Watch report said Israel committed war crimes and crimes against humanity by deliberately causing mass displacement of Palestinians in Gaza. Israel said the report was “completely false and out of touch with reality.”
About 1.9 million people – 90% of Gaza’s population – have fled their homes in the past year, and 79% of the territory is under an evacuation order issued by Israel, according to the UN.
Israel launched a campaign to destroy Hamas in response to the group’s unprecedented attack on southern Israel on October 7, 2023, in which approximately 1,200 people were killed and 251 people were taken hostage.
Nearly 44,000 people have been killed and more than 104,000 wounded in Gaza since then, according to the Hamas-run territory’s health ministry.
on wednesday The US has blocked a draft resolution on a cease-fire in Gaza in the UN Security Council – the fourth time it used its veto power during the conflict to protect its ally, Israel.
Fourteen of the council’s 15 members voted in favor of the draft, which demanded that the war in Gaza “must be immediately, unconditionally and permanently ended, and that all remaining hostages must be immediately and unconditionally released.”
Deputy US ambassador to the UN, Robert Wood, said the document “abandons” the need for “a link between the ceasefire and the release of the hostages”.
Wood said the proposed resolution would send a “dangerous message” to Hamas that “there is no need to return to the negotiating table.”
In a separate development, US mediator Amos Hochstein arrived in Israel from Lebanon.
He said he saw a “real opportunity” to end the conflict in Lebanon after the Lebanese government and Hezbollah largely agreed to a U.S. ceasefire proposal.
