Palestinian medical officials reported at least 21 people killed in fighting Tuesday throughout Gaza, though the Israeli army suggested the number could be higher.

One resident, Khader Mussa, said he fled his house while waving a white flag as the Israeli forces advanced. He spent the night huddling in the basement of a relative with 25 other people, including his pregnant wife and his parents.

"Thank God we survived this time and got out alive from here. But we don't know how long we'll be safe in my brother's home," Mussa, 35, told The Associated Press by telephone.

Several buildings were on fire, witnesses said, including a lumberyard. The sounds of battle could be heard clearly around the city of 400,000 as the Israeli forces, backed by artillery and attack helicopters, moved in. Israeli gunboats shelled the coast from the west.

The Israeli military said it carried out some 60 airstrikes overnight, hitting groups of Hamas militants holed up in a hotel, a house and a mosque. It said it also struck 15 squads of gunmen, rocket launching sites and 15 smuggling tunnels along the Egyptian border.

The army said it had killed or wounded about 30 militants, and that three soldiers were wounded in overnight fighting. Among them was an officer who was seriously wounded when a bomb exploded in a northern Gaza house that he was searching. Weapons, including a machine gun, were later found in the house, the military said.

Dr. Moaiya Hassanain, a Palestinian Health Ministry official, said dozens of calls for ambulances had been received, but they could not be dispatched because of the fighting.

The Gaza fighting has raised tensions around the region and galvanized anger toward Israel throughout the Arab world. On Tuesday, at least one gunman opened fire at an Israeli army patrol along the desert border between Israel and Jordan, the military said. There were no casualties, and Jordan said the claim was "baseless."

There was a similar shooting on the Israel-Syria border on Sunday, and last week militants in Lebanon fired rockets into an Israeli town in an apparent attempt to draw Israel into a second front.

The Israeli military has tightly controlled information from the battlefield, but indications have been that Hamas has not put up a serious fight.

Of the nine Israeli soldiers killed during the offensive, four were killed in "friendly fire incidents," a military inquiry concluded. Repeated Hamas claims of spectacular attacks on the Israelis have turned out to be false.

Speaking in parliament on Tuesday, Israel's military chief said his troops have achieved a lot but "still have work to do" in fighting Hamas in Gaza.