Thousands of Palestinian residents of the northern Gaza Strip fled their homes and sought safety in United Nations shelters.
They were heeding warnings from the Israeli military about impending plans to bomb the area in the sixth day of an offensive against Hamas that has killed more than 160 people.
The fighting showed no signs of slowing, despite international calls for a ceasefire and growing concerns about the mounting civilian death toll in Gaza.
US Secretary of State John Kerry spoke to Israeli Prime Minister Benjamin Netanyahu and voiced US “readiness” to help restore calm, while Egypt, a key mediator between Israel and Hamas, continued to work behind the scenes.
UN Secretary-General Ban Ki-moon called for an immediate ceasefire, saying he “strongly believes that it is in the interest of both sides that steps toward dangerous escalation be replaced with immediate measures to end the fighting, thus preventing further casualties and greater risks to regional peace and security”.
He condemned Hamas’s indiscriminate firing of rockets against Israeli civilian targets as “a violation of international law”.
Amid the diplomacy, Israel said it was pushing forward with preparations for a possible ground invasion of Gaza. Thousands of troops have massed along the border in recent days.
“We don’t know when the operation will end,” Mr Netanyahu told his Cabinet. “It might take a long time.” He said the military was prepared “for all possibilities”.
Israel launched the offensive last Tuesday in what it said was a response to heavy rocket fire out of Hamas-controlled Gaza.
Early yesterday, the Israeli air force dropped leaflets around the northern Gaza town of Beit Lahia ordering people to evacuate their homes.
Israel says much of the rocket fire has come from the area, and the military carried out a brief ground operation on what it said was a rocket-launching site that could not be struck from the air. Four Israeli soldiers were slightly wounded before returning to Israel.
The UN refugee agency for Palestinians, UNRWA, said some 17,000 Palestinians had headed to special shelters set up in 20 United Nations schools in Gaza.
“The fact that in a span of almost a few hours, 10,000 people sought refuge in these 15 schools is an indication to the difficult situation on the ground,” said Sami Mshasha, a UNRWA spokesman.
Some raced by in pick-up trucks, waving white flags. “Once we received the message, we felt scared to stay in our homes. We want to leave,” said one resident, Mohammed Abu Halemah.
Shortly before nightfall, Israel carried out a series of airstrikes in the northern Gaza town of Beit Lahia. Hamas’s Al-Aqsa TV station reported four air strikes in a 10-minute span, and a large plume of black smoke could be seen over the area from the Israeli border. There were no immediate reports of casualties.
Hamas, an Islamic militant group sworn to Israel’s destruction, has remained defiant, and it continued to fire rockets into Israel throughout the day.
It urged people in northern Gaza to stay in their homes and has so far rejected proposals for a ceasefire as unsatisfactory.
Moussa Abu Marzouk, a senior Hamas official, said: “They started the battle, and we will stay on our land and fight to protect our future.”