The Hamas-run health authority in Gaza on Monday said 45 people were killed and dozens injured in an Israeli airstrike that hit tents housing displaced people near the southern city of Rafah.
Most of the victims of the airstrike were women and children, the health authority said, describing the incident as a "massacre."
The information could not initially be independently verified, but the Palestinian Red Crescent earlier said on the social media platform X that there were "numerous" people killed and injured in the bombardment north-west of Rafah.
Prime Minister Benjamin Netanyahu told the Israeli parliament, the Knesset, that Israel was investigating "an incident."
"In Rafah, we have evacuated about one million civilians," Netanyahu said on Monday.
"Tragically, despite our immense efforts to avoid harming non-combatants, an incident occurred yesterday. We are investigating it thoroughly and will learn from it, as is our policy and longstanding conduct."
"For us, any non-combatant hurt is a tragedy, for Hamas, it is a strategy," Netanyahu stated. "That is the core difference."
Earlier, the Israel Defense Forces (IDF) said via X that two senior officials of the Palestinian militant group Hamas were killed in the airstrike.
"The strike was carried out against legitimate targets under international law, through the use of precise munitions and on the basis of precise intelligence that indicated Hamas' use of the area," the IDF said in a statement.
"The IDF is aware of reports indicating that, as a result of the strike and fire that was ignited, several civilians in the area were harmed. The incident is under review," the statement added.
The strike came just days after the International Court of Justice (ICJ) in The Hague ordered Israel to halt its assault on Rafah.
The incident sparked international horror and outrage.
The UN Security Council will hold an emergency meeting on Tuesday, following a request by Algeria supported by Slovenia, diplomats said.
UN Secretary-General António Guterres condemned the airstrikes "which killed scores of innocent civilians who were only seeking shelter from this deadly conflict," he wrote on X.
"There is no safe place in Gaza. This horror must stop."
Meanwhile, an exchange of fire between Israeli and Egyptian troops near the border to the Gaza Strip also caused new concerns that the war could escalate.
An Egyptian soldier was killed, an Egyptian military spokesman confirmed on Monday.
It is the first publicly known fatality in the ranks of the Egyptian military since the start of the Gaza war almost eight months ago. Israel's army confirmed an exchange of fire and said the incident was being investigated.
The situation at the Rafah border crossing has become increasingly tense. Israeli troops recently took control of the crossing on the Palestinian side as well as a border strip between Egypt and Gaza.
Aid organizations report dozens dead in Israeli strike
The Palestinian Red Crescent said that the air raid hit a designated humanitarian zone for those who had been forced to evacuate Rafah due to the Israeli fighting.
The aid organization Doctors Without Borders (MSF) reported that a medical facility it supports in the area treated dozens of wounded, while more than 15 dead people were brought to their trauma stabilization point.
"We are horrified by this deadly event, which shows once again that nowhere is safe. We continue to call for an immediate and sustained ceasefire in Gaza," MSF said on X.
Arab countries react angrily
Several Arab countries on Monday condemned the Israeli airstrike.
The Egyptian Foreign Ministry said the "deliberate bombing of displaced people's tents in the Palestinian city of Rafah" was a "new and blatant violation of the provisions of international law."
Qatar also condemned the attack as a "grave violation of international laws that will aggravate the humanitarian crisis in the besieged Strip."
The Qatari Foreign Ministry expressed concern that the latest strike would complicate ongoing mediation efforts and obstruct a permanent ceasefire agreement in the Gaza Strip.
Hamas confirmed to dpa that it was suspending ceasefire negotiations which were expected to restart this week.
In separate statements, Jordan and Kuwait condemned "war crimes" committed by Israel in Gaza and urged the international community to compel Israel to adhere to the ICJ ruling on Rafah.
Germany calls strike 'a mistake'
German government spokesman Steffen Hebestreit said the airstrike was presumably "a mistake" and again defended Israel's "right to defend itself within the framework of international law."
Asked about allegations of Israeli war crimes in Gaza, Hebestreit said that the German government would withhold judgement.
"The conclusion as to whether this is a war crime in terms of international law is something that must be left to lawyers who know the exact facts," Hebestreit said in Berlin.
If there is evidence of such a crime, the German government would certainly condemn it, Hebestreit said.
World leaders condemn Israeli airstrike in Rafah
French President Emmanuel Macron was among the leaders calling for an immediate ceasefire in Gaza and full compliance with international law.
"These operations must stop," wrote an "outraged" Macron on X. "There are no safe zones for Palestinian civilians in Rafah."
The "devastating images" following the attack were "heartbreaking," a US State Department spokesperson said.
"We are actively engaging the IDF and partners on the ground to assess what happened, and understand that the IDF is conducting an investigation."
"We are horrified by strikes that killed Palestinian civilians in Rafah," Canadian Foreign Minister Mélanie Joly wrote on X.
"Israel's strikes have had horrific and unacceptable consequences," Australia's Foreign Minister Penny Wong wrote.
The war in Gaza was triggered by the massacre of more than 1,200 people by terrorists from Hamas and other militant groups on October 7.
Israel responded with massive airstrikes and a ground offensive. International criticism of Israel has grown as the death toll among Palestinians has increased. It currently stands at over 35,000 people killed, according to Hamas authorities in Gaza.
