Israeli airstrikes killed at least 100 Palestinians across the Gaza Strip on Thursday, including 27 or more sheltering at a school, according to Palestinian medical authorities, in a stepped-up offensive that Israel’s military said is intended to pressure Hamas and eventually expel the militant group.

The bodies of 14 children and five women were recovered from the school in the Tuffah neighborhood of Gaza City and the death toll could rise because some of the 70 wounded sustained critical injuries, said Health Ministry spokesman Zaher al-Wahidi. More than 30 other Gaza residents were killed in strikes on homes in the nearby neighborhood of Shijaiyah, he said, citing records at Ahli Hospital.

The Israeli military said it struck a “Hamas command and control center” in the Gaza City area, and said it took steps to lessen harm to civilians. Israel gave the same reason — striking Hamas militants in a “command and control center” — for attacking a United Nations building used as a shelter a day earlier, killing at least 17 people.

Hamas called the strike on the school a “ heinous massacre” of innocent civilians.

The strikes came as Israel’s military ordered more residents in parts of northern Gaza to move west and south to shelters, warning that it planned to “work with extreme force in your area.” A number of the Palestinians leaving the targeted areas did so on foot, with some carrying their belongings on their backs and others using donkey carts.

“My wife and I have been walking for three hours covering only one kilometer,” said Mohammad Ermana, 72. The couple, clasping hands, each walked with a cane. “I’m searching for shelters every hour now, not every day,” he said.

Israel has issued sweeping evacuation orders for parts of northern Gaza ahead of expected ground operations. The U.N. humanitarian office said around 280,000 Palestinians have been displaced since Israel ended the ceasefire with Hamas last month.

The fresh evacuation orders came a day after senior government officials said Israel would seize large parts of the Palestinian territory and establish a new security corridor across it. To pressure Hamas, Israel has imposed a monthlong blockade on food, fuel and humanitarian aid that has left civilians facing acute shortages as supplies dwindle — a tactic that rights groups say is a war crime.

Hamas says it will only release the remaining 59 hostages — 24 of whom are believed to be alive — in exchange for the release of more Palestinian prisoners, a lasting ceasefire and an Israeli pullout from Gaza. The group has rejected demands that it lay down its arms or leave the territory.

Another deadly day in Gaza war zones

Overnight strikes by Israel killed at least 55 people in the Gaza Strip, hospital officials said Thursday.

In the southern city of Khan Younis, officials said the bodies of 14 people had been taken to Nasser Hospital – nine of them from the same family. The dead included five children and four women. The bodies of another 19 people, including five children aged between 1 and 7 years and a pregnant woman, were taken to the European hospital near Khan Younis, hospital officials said. In Gaza City, 21 bodies were taken to Ahli hospital, including those of seven children.

Later in the day, strikes killed four more people in Khan Younis, according to Nasser Hospital, and another two people were killed in central Gaza and taken to Al Aqsa Hospital.

The attacks came as the Israeli military promised an independent investigation of a March 23 operation in which its forces opened fire on ambulances in southern Gaza. U.N. officials say 15 Palestinian medics and emergency responders were killed, and their bodies and ambulances were buried by Israeli soldiers in a mass grave.

The military initially said the ambulances were operating suspiciously and that nine militants were killed. The military said the probe would be led by an expert fact-finding body “responsible for examining exceptional incidents” during the war. Rights groups say such investigations are often lacking and that soldiers are rarely punished.

The head of the Palestine Red Crescent Society, Younes Al-Khatib, said Thursday he believed some of the medics were still alive when they were overtaken by Israeli forces. The organization’s radio dispatchers heard a conversation in Hebrew between medics and Israeli soldiers after the ambulances had come under fire, Al-Khatib told members of the U.N. Security Council.

Deadly Israeli strike reported in Lebanon

At least two people were killed early Friday in an apparent Israeli airstrike that hit an apartment in a multistory building in the coastal city of Sidon in Lebanon. An Associated Press photographer at the scene saw two bodies being carried out of the building by emergency responders.

There was no immediate statement from the Israeli military.

It was the first time an airstrike had hit Lebanon’s third largest city since a tenuous ceasefire agreement brought an end to the latest Israel-Hezbollah war in late November. Israel has continued to carry out regular airstrikes targeting what it has said are facilities and officials of Hezbollah and allied groups since the ceasefire.