Israeli air attacks on the Hamas-ruled Gaza strip entered a third consecutive day, with more than 300 Palestinians dead in the first two days of the raids, including at least 51 civilians.
Israel said it was preparing to launch a ground invasion if necessary, and had called up 6500 reserve troops.
Air force jets bombed the Islamic University in the Gaza Strip, a significant cultural symbol for Hamas, and also struck centres of Hamas strength, including security compounds, government offices and tunnels into Egypt that provide a lifeline for Gaza under Israel's blockade.
The air raids are aimed at forcing Palestinian militants to halt rocket fire into southern Israel. Israel said 110 rockets had been fired into Israel since Saturday.
The Palestinian president, Mahmoud Abbas, said the Israeli attacks could have been avoided if Hamas had extended its ceasefire with Israel after it expired on 19 December.
There have been demonstrations against the Israeli action across the Arab world and in several capitals in Europe, with thousands of protestors taking to the streets.
Egypt has demanded an end to the attacks and Syria has suspended indirect peace talks with Israel. Earlier, the United Nations Security Council called for an immediate end to the violence.