Facing a possible Israeli ground invasion, militants warned international airlines they would fire rockets at Tel Aviv's main airport. A rocket also caused the first serious Israeli casualty - one of eight people hurt when a fuel tanker was hit at a service station in Ashdod, 30km (20 miles) north of Gaza.
Medical officials in Gaza said at least 74 civilians, including 23 children, were among those killed in the unrelenting aerial bombardments that Israel began on Tuesday.
A day after US President Barack Obama told Israeli Prime Minister Benjamin Netanyahu he was willing to help negotiate a ceasefire, Palestinian President Mahmoud Abbas urged the United Nations to order an immediate truce.
But Israel said it was determined to end cross-border rocket attacks that intensified last month after its forces arrested hundreds of activists from the Islamist Hamas movement in the occupied West Bank following the abduction there of three Jewish teenagers who were later found killed. A Palestinian youth was killed in Jerusalem in a suspected Israeli revenge attack.
Israel's military commander, Lieutenant-General Benny Gantz, said his forces were ready to act as needed - an indication of a readiness to send in tanks and other ground troops, as Israel last did for two weeks in early 2009. "We are in the midst of an assault and we are prepared to expand it as much as is required, to wherever is required, with whatever force will be required and for as long as will be required," Lt Gen Gantz told reporters.
Western-backed Mr Abbas, who is based in the West Bank and agreed a power-sharing deal with Gaza's dominant Hamas in April after years of feuding, called for UN help: "The Palestinian leadership urges the Security Council to quickly issue a clear condemnation of this Israeli aggression and impose a commitment of a mutual ceasefire immediately," he said.
After the failure of the latest US-brokered peace talks with Israel, Mr Abbas's accord with Hamas angered Israel.
The rocket salvoes by the hardline movement and its allies, some striking more than 100 km (60 miles) from Gaza, have killed no-one so far, due in part to interception by Israel's partly-US funded Iron Dome aerial defence system. But racing for shelter had become a routine for hundreds of thousands of Israelis, and their leaders have hinted they could order troops into the Gaza Strip. Some 20,000 reservists have already been mobilised.
Hamas's armed wing said it would fire rockets at Tel Aviv's Ben-Gurion international airport and warned airlines not to fly to Israel's main gateway to the world.
The airport has been fully operational since the Israeli offensive began, with no reports of rockets from Gaza - largely inaccurate projectiles - landing anywhere near the facility, which is within an area covered by Iron Dome.
"The armed wing of the Hamas movement has decided to respond to the Israeli aggression, and we warn you against carrying out flights to Ben-Gurion airport, which will be one of our targets today because it also hosts a military air base," said a statement by Hamas's Izz el Deen al Qassam Brigades.