A Palestinian health official says 13 people were killed early Wednesday after tank shells hit a U.N. school in Gaza where hundreds of Palestinians had taken refuge from Israeli attacks.
Gaza health official Ashraf al-Kidra said another 40 Palestinians were wounded in the shelling.
Adnan Abu Hasna, a spokesman for a U.N. aid agency, said tank shells hit the Abu Hussein U.N. school in the Jabaliya refugee camp around 4:30 a.m.
An Israeli army spokeswoman said she was checking for details.
Eight people, including five members of the same family, were killed in other strikes.
The bloodshed came hours after Hamas denied agreeing to a 24-hour cease-fire announced Tuesday by the Palestinian Liberation Organization, with the group's armed wing, Al Qassam, saying there will be no peace until Israel lifts its blockade of the occupied Gaza Strip.
Yasser Abed Rabbo, a PLO executive committee member, said earlier Tuesday that all Palestinian factions offered a 24-hour cease-fire in Gaza and that a unified delegation had been sent to Cairo to talk about the next steps.
But Hamas spokesman Sami Abu Zuhri denied that the group agreed to the truce, with the Palestinian news agency Maan quoting him as saying that "Yasser Abed Rabbu's statement that Hamas agreed to a cease-fire for 24 hours is not true and has nothing to do with the resistance's stand."
"We will consider a cease-fire when Israel commits to it with international guarantees," Abu Zuhri added. Hamas has said its conditions for a truce include Israel's ending its siege of Gaza and the release of long-serving political prisoners that was agreed to in a 2012 cease-fire but not carried out completely.
Though Israel pulled out its illegal settlements, checkpoints and military from inside Gaza in 2005, it remained in control of the territory's land borders, air space, and sea — effectively strangling the coastal enclave, with a devastating effect on Gaza's economy and infrastructure.
Israel has said it will not comment until Hamas officially presents an offer. Haaretz, an Israeli news website, reported senior Israeli officials as saying no cease-fire had been agreed upon, though negotiations were taking place in Cairo.
Word of the possible cease-fire came after Israel's military pounded the Gaza Strip overnight on Monday into Tuesday, killing at least 100 people in assaults launched from air, land and sea after midnight, according to Palestinian officials.
Five families were almost entirely wiped out in Khan Younis and Rafah, according to reports by Haaretz. For many Gaza residents, it was the heaviest night of bombing since the military offensive against the territory began three weeks ago.
Gaza's only power plant was shut down overnight because of a large fire due to Israeli shelling, Palestinian officials said.
Fathi al-Sheikh Khalil, deputy head of the power authority in Gaza, said that a shell struck a fuel container and ignited a fire and that another shell damaged a steam engine in the plant, Maan reported. Gaza residents already faced severe power shortages due to Israel's years-long blockade of the territory, which has resulted in fuel and spare parts shortages.
Running water and sewage treatment will be affected by the downed plant, as both services need electricity to function, so many homes will be without water or power, and untreated sewage could run into the Mediterranean Sea.
Since the military offensive began on July 8, more than 1,258 Palestinians — most of them civilians — have died, and more than 7,100 others have been wounded. Israeli fatalities have risen to 55, including two civilians. A Thai foreign worker has also been killed in the violence.
Israeli aircraft fired a rocket at the house of Ismail Haniyeh, Gaza's senior Hamas leader, before dawn on Tuesday in the Shati refugee camp. No casualties were reported, according to Gaza's Interior Ministry.
"My house is not dearer than any of the houses of our people," Haniyeh was quoted as saying on a Hamas website. "The destruction of stones will not break our will, and we will continue our resistance until we gain freedom."
An Israeli military spokeswoman had no information on the report but was checking for details. Hamas broadcast outlets Al-Aqsa TV and Al-Aqsa Radio were also targeted, and the radio station went silent. The television station continued to broadcast.
In a separate development on Tuesday, the United Nations agency that looks after Palestinian refugees said it had found a cache of rockets at one of its schools in the Gaza Strip.
United Nations Relief and Works Agency (UNRWA) spokesman Chris Gunness said in a statement, "We condemn the group or groups who endangered civilians by placing these munitions in our school. This is yet another flagrant violation of the neutrality of our premises. We call on all the warring parties to respect the inviolability of UN property."
The overnight strikes came after a day of heavy Hamas-Israeli fighting. The Hamas-run Health Ministry said 10 people — including nine children under the age of 12 — were killed and 46 wounded in a blast at a park in the Shati refugee camp on the outskirts of Gaza City. Each side blamed the tragedy on the other.
Meanwhile, the Israeli military said five soldiers died in a gun battle with fighters who crossed into Israel via a tunnel near the community of Nahal Oz, close to the border with the Gaza Strip. Hamas said its forces infiltrated Israel to retaliate for the killing of children in a beach camp.
In a televised address on Monday night, Israeli Prime Minister Benjamin Netanyahu said any solution to the crisis would require the demilitarization of the Palestinian territory, controlled by Hamas and its allies.
"We will not finish the mission, we will not finish the operation without neutralizing the tunnels, which have the sole purpose of destroying our citizens, killing our children," he said, adding that it was a "painful day."
"We need to be prepared for a protracted campaign. We will continue to act with force and discretion until our mission is accomplished," Netanyahu said.
As he spoke, the Israeli military sent messages to thousands of Palestinians in Shujaiya, Zeitun, Jebaliya, Beit Lahiya and Beit Hanun, urging them to flee their homes and seek shelter in central Gaza City.
The U.N. said on Monday that more than 167,000 displaced Palestinians had taken shelter in its schools and buildings, heeding the Israeli evacuation warnings.
Many Jebaliya residents said they did not dare attempt an escape. Sufian Abed Rabbo said his extended family of 17 took refuge under the stairway in their home.
"God help us. We have nothing to do but pray," the 27-year-old told The Associated Press by phone. "I don't know who left and who stayed, but in our street, we are all very scared to move."
Israeli tanks shelled border areas of Gaza, killing five people, including three children and a 70-year-old woman, and wounding 50 in Jebaliya, the Red Crescent said.
A number of rockets from Gaza were launched toward southern and central Israel, including the Tel Aviv area. At least one rocket was intercepted by the Iron Dome system. No casualties or damage were reported.
Israel's Channel 10 television reported on an opinion poll that showed overwhelming public support for continuing the Gaza offensive until Hamas is "disarmed." However, foreign pressure has been building on Netanyahu to show restraint.
President Barack Obama and the U.N. Security Council have called for an immediate cease-fire to allow relief to reach Gaza's 1.8 million Palestinians, followed by negotiations on a more durable cessation of hostilities.
Israel has said it wants guarantees that Hamas will be stripped of its tunnels and missile stocks. In his television address Netanyahu said any solution to the crisis would need to see Palestinian fighters stripped of their weapons.
"The process of preventing the armament of the terror organization and demilitarization of the Gaza Strip must be part of any solution. And the international community must demand this forcefully," he said.
Hamas, for its part, has said it wants international guarantees that Israel will abide by the terms of any cease-fire agreed upon, citing Israel's failure to adhere to the terms of the last truce in 2012.
Hamas' Abu Zuhri said, "[Netanyahu's] threats do not frighten either Hamas or the Palestinian people, and the [Israeli] occupation will pay the price for its massacres against children and civilians."
Tensions between the Israelis and Palestinians have been rising since April, when Hamas' unity deal with Fatah led Israel to suspend the last round of peace talks. In June three Israeli teenagers who were studying in illegal West Bank settlements were abducted and later found killed. Israel blamed Hamas and began a crackdown on the group in the West Bank.
Hamas denied having any involvement in the crime. The perpetrators were later found to be unaffiliated with Hamas leadership and acted as part of a lone cell, Israeli police spokesman Micky Rosenfeld told a BBC journalist last week.
On Tuesday, Israeli-Arab Knesset member Haneen Zoabi was barred from parliamentary sessions for six months because of her comments on the teens' deaths, Haaretz reported.
Zoabi said in an interview on Radio Tel Aviv that the perpetrators of the crime were not terrorists but "people who don't see any way to change their reality, and they are forced to use these means until Israel will wake up a little, until Israeli citizens and society will wake up and feel the suffering of the other."
Al Jazeera and wire services