At least 18 people have died in the Gaza Strip as Israeli forces kept up air strikes they say are aimed at stopping rocket attacks into Israel.
Israeli officials said only one rocket was fired overnight, although some rocket fire resumed on Monday morning.
More than 90 Palestinians and three Israelis have died since Operation Pillar of Defence began on Wednesday.
The UN secretary general has called for an immediate ceasefire as he prepares to join truce talks in Egypt.
Israeli Prime Minister Benjamin Netanyahu said on Sunday he was ready to expand the operation, after Israel authorised the mobilisation of up to 75,000 army reservists.
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Egyptian President Mohammed Mursi has said an Israeli ground invasion would have "serious repercussions", saying Egypt would never accept it "and neither will the free world".
The Arab League, which met in emergency session in Cairo on Sunday, is sending a delegation of foreign ministers to Gaza on Tuesday.
Israel's Defense Forces (IDF) said it had targeted around 80 sites overnight into Monday, including militant-owned buildings, weapons storage facilities and police stations, bringing its total to 1,350 sites targeted since Wednesday.
One of the overnight blasts destroyed a Hamas police headquarters.
As well as air strikes there was the rhythmic thud of shelling from Israeli warships, says the BBC's Jon Donnison in Gaza City.
On Monday morning, a rocket fired from Gaza hit a school in the Israeli city of Ashkelon, said the IDF. Classes had already been cancelled.
At least nine children were killed in Gaza on Sunday - the bloodiest day so far - and TV reports showing horrific images of their burned and bloodied bodies have been fuelling Palestinian anger, adds our correspondent.
In one strike nine members of the family of Hamas policeman Mohamed Dalou were killed.
Israel's Haaretz newspaper quoted IDF officials as saying the strike had hit a neighbour's house by mistake because of a "technical error". The IDF said it did not know what the Haaretz source was, but was looking into the report.
The army's chief military spokesman, Yoav Mordechai, told Israel's Channel 2 TV that the intended target of the strike had been Yehiya Rabiah, the head of Hamas's rocket-launching unit, but that there had been "civilian casualties".
UN Secretary-General Ban Ki-moon said he was deeply saddened by the deaths of the Dalou family and other Palestinians, while expressing alarm at the continued rocket fire from Gaza against Israeli towns.
"This must stop," he said in a statement released late on Sunday. "I strongly urge the parties to co-operate with all efforts led by Egypt to reach an immediate cease-fire."
Hamas's military wing said earlier: "The massacre of the Dalou family will not pass without punishment."
The charity Save the Children has said families in Gaza are running out of food and water, with most trapped in their homes, enduring power cuts of up to 18 hours a day.
War or peace?
Egypt has been leading efforts to broker a peace deal, with both senior Israeli and Hamas officials in Cairo for talks.
The BBC's Middle East editor, Jeremy Bowen, says he fears the conflict might get worse before any truce is agreed.
Since the conflict began, Gaza militants have fired 848 rockets towards Israel - 546 hit Israel, 302 were intercepted by the Iron Dome interceptions, the IDF says.
Before the recent offensive, Israel had repeatedly carried out air strikes on Gaza as Palestinian militants fired rockets across the border.
But the aerial and naval bombardment is its most intense assault on the territory since Israel launched a full-scale invasion four years ago.
Hamas seized control of Gaza in 2007, a year after winning a decisive victory in general elections. Israel withdrew from the strip in 2005 but maintains a blockade around it.
Israel, as well as the United States and the European Union, regards Hamas as a terrorist organisation.