Around 500 Palestinians have been killed in a blast at a Gaza hospital that Palestinian health authorities said was caused by an Israeli air strike.
The blast was the bloodiest single incident in Gaza since Israel launched an unrelenting bombing campaign against the densely populated territory in retaliation for a deadly cross-border Hamas assault on southern Israeli communities on 7 October.
The Israeli military has denied responsibility for the hospital attack, and has blamed a failed rocket launch by a Palestinian militant group.
A spokesperson for the Islamic Jihad militant group in Gaza denied the Israeli accusations.
The health minister in the Hamas-run government of Gaza, Mai Alkaila, accused Israel of a massacre. A Gaza civil defence chief said 300 people were killed and a health ministry official said 500 were killed.
Lebanon's Hezbollah group denounced what they said was Israel's deadly attack, and announced that Wednesday would be "a day of unprecedented anger" against Israel and Joe Biden's visit to the country.
Tens of thousands of families have flocked to Gaza's overwhelmed hospitals seeking refuge from seemingly endless Israeli army shelling.
"The hospital was housing hundreds of sick and wounded, and people forcibly displaced from their homes" because of other strikes, a statement said.
Footage from the scene showed medics and civilians recovering bodies with white bags or blankets. Bloodstains and multiple torched cars were visible in the dark hospital courtyard.
Palestinian president Mahmoud Abbas has declared three days of mourning after the attack, calling it "genocide" and a "horrific crime".
Mr Abbas announced "public mourning for three days and flags flown at half-mast for the martyrs of the Baptist hospital massacre and all our people's martyrs", the official Palestinian news agency Wafa said.
Canada, Iran, Turkey, Qatar, and Jordan are among just some of the countries which have strongly condemned the attack, with EU chief Ursula von der Leyen also saying that attacking civilian infrastructure was outside international law.
Taoiseach Leo Varadkar has repeated his insistence that "civilian infrastructure" must not be targeted in Gaza.
Speaking at Government Buildings after a remote meeting of European leaders this evening, Mr Varadkar said: "We don't have the full information yet, but I think it emphasises very much our view that civilian infrastructure should not be targeted."
Around 3,000 Gazans have been killed in Israeli strikes since the country's military started bombarding the Palestinian territory on 7 October.
The conflict erupted when Hamas militants from Gaza launched attacks on multiple border communities and military posts in Israel.
More than 1,400 people have been killed in Israel during the war, while the army has identified 199 hostages abducted to Gaza.
The hospital attack comes after at least six people were killed when a UN-run Gaza school they were sheltering in was hit during Israeli air strikes today, the United Nations agency for Palestinian refugees said.
"At least six people were killed this afternoon when an UNRWA school was hit in Al-Maghazi refugee camp", in central Gaza, the UN agency said.
"The school was hit during Israeli forces air strikes and bombardment on the Gaza Strip," it said.
Dozens of people, including UNRWA staff, were wounded and the school suffered severe structural damage, it said, adding the number of casualties was expected to rise.
"At least 4,000 people have taken refuge in this UNRWA school turned shelter. They had and still have nowhere else to go," the agency said.
"This is outrageous, and it again shows a flagrant disregard for the lives of civilians," it added.
United Nations Relief and Works Agency - or UNRWA - was established in 1949 to provide humanitarian assistance and protection to Palestinian refugees.
It operates in annexed east Jerusalem, Gaza, the West Bank, Jordan, Lebanon and Syria.
Israel has imposed a total blockade on the enclave, halting food, fuel and medical supplies, which are rapidly running out.
Scores of trucks carrying vital supplies for Gaza headed towards the Rafah crossing in Egypt today, the only access point to the coastal enclave outside Israel's control, but there was no clear indication that they would be able to enter.
It comes as US President Joe Biden will visit Israel tomorrow to show support for its war on Hamas after Washington said Prime Minister Benjamin Netanyahu had agreed to let humanitarian aid reach besieged Gazans.
After Israel, Mr Biden is expected to travel to Jordan to meet King Abdullah and Egyptian President Abdel Fattah al-Sisi.
Mr Abbas canceled a planned meeting with Mr Biden, following the strike on the hospital, a senior Palestinian official said.
Mr Abbas is the president of the Palestinian Authority which exerts limited self rule in the Israeli-occupied West Bank but lost control of Gaza to Hamas in 2007. The PA has accused Israel of genocide and ethnic cleansing in Gaza.
The World Health Organization says that it needs urgent access to Gaza to deliver aid and medical supplies and avert a human catastrophe.
The UN body says it estimates that 2,800 Palestinians have been killed and 11,000 injured since Israeli air strikes commenced.
UNRWA said only around 14% of Gazans had access to water through a single pipe to Khan Younis that Israel allowed to open for three hours yesterday.
Concerns about dehydration and diseases were high as water and sanitation services had collapsed.
"People will start dying without water," UNRWA said.
Watch: The Israel-Palestine conflict - a brief history
Read more: Dublin woman says 28 relatives killed in Israeli air strikes on Gaza
Clashes in Israel's north
Cross-border fighting has also intensified on a second front on Israel's northern border with Lebanon. Clashes there have been the deadliest since the last full-blown war between Israel and Lebanon's Hezbollah group 17 years ago.
The Israeli military has said that it had killed four people who had tried to cross the border to plant explosives. Security sources in Lebanon said four people had been killed by Israeli shelling near the village of Alma Al-Shaab on the Lebanese side of the frontier.
Israel ordered the evacuation yesterday of 28 of its villages near the Lebanese border.
Iran, which sponsors both Hamas and Hezbollah, has celebrated the Hamas attacks on Israel but denies being behind them. The country's foreign minister Hossein Amir-Abdollahian told state TV that "pre-emptive action" would be taken against Israel in the coming hours.
"We cannot be indifferent to the war crimes committed against the people of Gaza," he said.
There has also been an intensification of deadly clashes in the Israeli-occupied West Bank, already engulfed in its worst unrest for years before the Hamas attacks from Gaza.
