At least 54 Palestinians and two Israeli soldiers have been killed in one of the deadliest days of fighting in Gaza since troops withdrew in 2005.
There is no sign the fighting is abating
Doctors said at least eight were children and up to 16 were militants. Israel said most were militants.
More than 150 Palestinians - and seven Israelis - have been injured. Israel says it wants to stop rocket attacks, but about 50 hit Israel on Saturday.
Palestinian leaders have called for international protection.
The UN Security Council is meeting in emergency session at the request of Palestinian Authority President Mahmoud Abbas.
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As the council went into closed session, Secretary General Ban Ki-moon condemned Israel for "excessive and disproportionate" use of force in the Gaza Strip.
He also condemned Palestinian rocket attacks against southern Israel.
The BBC's Laura Trevelyan at the UN says diplomats will have problems agreeing a statement on the crisis that is acceptable to both the Arab grouping and the US - a staunch ally of Israel.
As the Security Council was assembling reports came in that an Israeli air strike in Gaza City had destroyed the empty offices of Hamas leader Ismail Haniya, prime minister in the territory's unrecognised government.
Battle of words
Mr Abbas earlier said the Israeli raids were "more than a holocaust".
Israel says it wants to end the agony caused by militant rockets
He was apparently alluding to controversial remarks made on Friday by Israel's Deputy Defence Minister Matan Vilnai, who said Palestinians risked a "shoah" - the Hebrew word for a big disaster as well as for the Nazi Holocaust.
Mr Vilnai's colleagues insisted he had not meant "genocide".
Khaled Meshaal, Hamas's exiled leader in Syria, went further, calling Israel's actions "the real holocaust".
The Israeli raids began after a rocket fired by Hamas militants killed an Israeli student in the southern town of Sderot, the first such death in nine months, on Wednesday.
More than 80 Palestinians have been killed since then.
In a statement on Saturday, Israel's Defence Minister Ehud Barak blamed Hamas for the killings.
"We are not happy about civilians being hurt in Gaza," Mr Barak said.
"Hamas and those who fire rockets at Israel are responsible and they will pay the price."
Israel has said it may launch a full-scale attack on Gaza in response to militant rocket attacks.
The BBC's Katya Adler in Jerusalem says Israel's leaders have been under pressure from some quarters to launch a ground invasion.
However, a recent opinion poll has indicated a majority of Israelis favour a truce with the Islamist movement Hamas, which controls Gaza.
Tanks and troops have made an incursion into northern Gaza, encountering resistance from Palestinian militants, as Israeli planes made several air raids.
On one occasion, a house east of the Jabaliya refugee camp was struck - two children, a brother and sister, were killed.
Later, a 15-year-old girl and her 16-year-old sister were also killed.
In another attack, a mother was killed as she was preparing breakfast for her children, medical workers said.
And a resident of Jabaliya told the Associated Press news agency that one of his relatives had been killed.
"His body is still lying on the ground," he said. "Ambulances tried to come, but they came under fire."