GAZA CITY, Gaza Strip (AP) -- Israel assassinated a Hamas strongman Thursday in its first assault on the top leadership of Gaza's rulers, escalating a crushing aerial offensive even as it declared it was ready to launch a ground invasion. The airstrike targeted the four-story apartment building that was home to 52-year-old Nizar Rayan, ranked among Hamas' top five decision-makers in Gaza. It also killed 12 other people including two of Rayan's four wives and four of his 12 children, Palestinian health officials said. The Muslim faith allows men to have up to four wives.
While intensifying its 6-day-old military offensive against Hamas in Gaza, Israel also appeared to be sounding out a possible diplomatic exit by demanding international monitors as a key term of any future truce.
Israel launched the offensive Saturday to crush militants who have been terrorizing southern Israel with rocket fire from Gaza.
The campaign began after more than a week of intense Palestinian rocket fire that followed the expiration of a six-month truce. Israeli warplanes have carried out some 500 sorties against Hamas targets, and helicopters have flown hundreds more combat missions, a senior Israeli military officer said Wednesday.
More than 400 Gazans have been killed and some 1,700 have been wounded, Gaza health officials said. The U.N. says the death toll includes more than 60 civilians, 34 of them children.
Three Israeli civilians and one soldier have also died in rocket attacks that have reached deeper into Israel than ever before, bringing one-eighth of the population within rocket range.
Israel has made clear that no one in Hamas is immune from attack and Thursday's strike drove that point home. It flattened Rayan's apartment building, sending a thick plume of smoke into the air and heavily damaged several neighboring buildings.
Hamas leaders went into hiding before Israel launched its operation, but Rayan was known for openly defying Israel.
A professor of Islamic law, Rayan was closely tied to Hamas' military wing and was respected in Gaza for donning combat fatigues and personally participating in clashes against Israeli forces. He sent one of his sons on an October 2001 suicide mission that killed two Israeli settlers in Gaza.
Throughout the day, huge blasts had rocked cities and towns across Gaza as Israeli warplanes went after Gaza's parliament building, militant field operatives, police and cars. The military said aircraft also bombed smuggling tunnels along the Gaza-Egypt border, part of an ongoing attempt to cut off Hamas' last lifeline to the world outside the embattled Palestinian territory.
So far, the campaign to crush rocket fire on southern Israel has been conducted largely from the air. But military spokeswoman Maj. Avital Leibovich said preparations for a ground operation were complete.
"The infantry, the artillery and other forces are ready. They're around the Gaza Strip, waiting for any calls to go inside," Leibovich said.
Hamas threatened to take revenge against Israeli soldiers who were massed along the border with Gaza, waiting for a signal to invade.
"We are waiting for you to enter Gaza to kill you or make you into Schalits," it said, referring to Sgt. Gilad Schalit who was seized by Hamas-affiliated militants 2- 1/2 years ago and remains in captivity.
Instead, they authorized the military to push ahead with its campaign against militants, who fired more than 30 rockets into Israel by late Thursday afternoon, according to the military. No injuries were reported, but an eight-story house in Ashdod, 23 miles from Gaza, was hit by a rocket that pierced through two floors.
Israeli Prime Minister Ehud Olmert told a meeting of mayors of southern communities Thursday that Israel would not shy from using its vaunted military power.
"We have no interest in a long war. We do not desire a broad campaign. We want quiet," Olmert said. "We don't want to display our might, but we will employ it if necessary."
Ordinary Israelis are not eager to see the operation expand beyond the air-based campaign, a poll Thursday showed.
Earlier this week, Olmert rebuffed a French proposal for a two-day suspension of hostilities. But at the same time, he seemed to be looking for a diplomatic way out, telling Secretary of State Condoleezza Rice and other world leaders that Israel wouldn't agree to a truce unless international monitors took responsibility for enforcing it, government officials said. They spoke on condition of anonymity because the talks were confidential.
International intervention helped Israel to accept a truce that ended its 2006 war with Lebanese Hezbollah guerrillas, when the U.N. agreed to station peacekeepers to enforce the terms. This time, Israel isn't seeking a peacekeeping force, but a monitoring body that would judge compliance on both sides.
The idea was floated before the offensive but did not gain traction because of the complications created by the existence of rival Palestinian governments in the West Bank and Gaza, defense officials said.
Gaza has been under Hamas rule since the militant group overran it in June 2007; the West Bank has remained under the control of moderate Palestinian President Mahmoud Abbas, who has been negotiating peace with Israel for more than a year but has no influence over Hamas. Bringing in monitors would require cooperation between the fierce rivals.
An Abbas confidant said the Palestinian president supports international involvement.
"We are asking for a cease-fire and an international presence to monitor Israel's commitment to it," Nabil Abu Rdeneh said.