As nightfall approached, the fighting showed no signs of slowing, and the rocket attack on the city of Rishon Lezion, by far the deepest target reached by Gaza militants so far, raised the likelihood of an even tougher Israeli response.

Defense officials say Israel is prepared to launch a ground invasion into Gaza if necessary. And Prime Minister Benjamin Netanyahu said the army was prepared for a "significant widening" of its Gaza offensive.

"No government would tolerate a situation where nearly a fifth of its people live under a constant barrage of rockets and missile fire, and Israel will not tolerate this situation," he said. "This is why my government has instructed the Israeli Defense Forces to conduct surgical strikes against the terrorist infrastructure in Gaza. And this is why Israel will continue to take whatever action is necessary to defend our people."

The fighting, the heaviest in four years, has brought life to a standstill on both sides of the border, with schools canceled and people huddled indoors.

Israel and Hamas have largely observed an informal truce for the past four years. But in recent weeks, the calm unraveled in a bout of rocket attacks on Israel from Gaza and retaliatory Israeli airstrikes. From Israel's perspective, Hamas escalated tit-for-tat fighting in recent days with a pair of attacks: an explosion in a tunnel along the Israeli border and a missile attack on an Israeli military jeep that seriously wounded four soldiers.

Israeli officials say they have not yet decided on whether to launch a ground invasion in Gaza, and it's not clear what would trigger it. But a successful strike on Tel Aviv, Israel's commercial and cultural capital, would mark a significant escalation.

Although the rocket in Rishon Lezion, some 60 kilometers, or 40 miles north of Gaza, landed harmlessly in an open area, it illustrated the significant capabilities that Hamas militants have developed in recent years. Gaza militants had previously hit the city in 2009 fighting as well.

Israel launched the offensive on Wednesday, killing the head of Hamas' militant wing and destroying dozens of rocket launchers. Israel has made special efforts to destroy launchers for Hamas' Iranian-made "Fajr" rockets, which are believed capable of flying even deeper into Israel.

Israel's military spokesman, Brig. Gen. Yoav Mordechai, says the air operation has delivered a "strong blow" to militants' launching sites.

In all, 15 Palestinians have been killed and more than 100 wounded in fighting on Wednesday and Thursday. Three Israelis were killed earlier Thursday when a rocket struck an apartment building in the southern town of Kiryat Malachi. The Israeli military says three soldiers were wounded in a separate rocket attack.

The military said its air campaign has hit 230 targets across Gaza, and its "Iron Dome" rocket defense system has intercepted some 90 incoming rockets.

Still, Palestinian militants continued to launch rockets into Israel throughout the day.