GAZA CITY, Gaza Strip — Israel dropped a one-ton bomb on the home of a Hamas strongman Thursday, killing him along with two wives and four children in the first attack on the top leadership of Gaza's rulers. As the aerial bombardment escalated, the army said it was also poised to launch a ground invasion.
Israel also appeared to be sounding out a possible diplomatic exit from the 6-day-old military offensive against Hamas by demanding international monitors as a key term of any future truce.
The bombing targeted 49-year-old Nizar Rayan, ranked among Hamas' top five decision-makers in Gaza. His four-story apartment building crashed to the ground, sending a thick plume of smoke into the air and heavily damaging neighboring buildings. It killed Rayan and 11 others, including two of his four wives and four of his 12 children, Palestinian health officials said. The Muslim faith allows men to have up to four wives.
Israel has made clear that no one in Hamas is immune in this offensive, and the strike that flattened Rayan's apartment building in the northern town of Jebaliya drove that message home.
"We are trying to hit everybody who is a leader of the organization, and today we hit one of their leaders," Israeli Vice Premier Haim Ramon said in a television interview.
Hamas leaders went into hiding before Israel launched the offensive on Saturday, but Rayan was known for openly defying Israel. He was seen earlier Thursday praying in a mosque, and the military said he had a tunnel under his house that could serve as an escape route.
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.
Defense officials said a one-ton bomb was used to attack Rayan's home, and that weapons stored inside set off secondary explosions. They spoke on condition of anonymity because they weren't authorized to speak to the media.
Israel has assassinated top Hamas officials in the past, including the group's paraplegic spiritual leader, Sheik Ahmed Yassin, who was killed in a wheelchair as he left a mosque in 2004. It had halted the practice during a recent six-month truce, which expired last month and collapsed into all-out violence last week.
Israel launched the offensive to crush militants who have been terrorizing southern Israel with rocket fire from Gaza since the truce expired.
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.
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.
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."
Thousands of soldiers were massed along the border with Gaza, backed by tanks and artillery. Along the border, the ground troops watched warplanes and attack helicopters flying into Gaza, cheering each time they heard the explosion of an airstrike.
Hamas threatened to take revenge against the Israeli soldiers massed along the border with Gaza.
"We are waiting for you to enter Gaza to kill you or make you into Schalits," the group said in a statement, referring to Sgt. Gilad Schalit who was seized by Hamas-affiliated militants 2-1/2 years ago and remains in captivity.
Israeli Cabinet ministers have been unswayed by international calls to end the violence.
Instead, they authorized the military to push ahead with its campaign against militants, who fired more than 30 rockets into Israel Thursday, the military said. No injuries were reported, but an eight-story house in Ashdod, 23 miles from Gaza, was hit by a rocket.
Israel's foreign minister, Tzipi Livni, was in Paris on Thursday to prepare for an upcoming Mideast visit by French President Nicolas Sarkozy to push for an end to the violence. She told reporters the offensive was launched "to change the equation" with Hamas. She said the operation has badly damaged the Islamic militant group.
"We affected most of the infrastructure of terrorism in Gaza Strip and the question (of) whether it's enough or not will be according to our assessment on a daily basis," Livni said.
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 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.
Abbas confidant Nabil Abu Rdeneh said the Palestinian president is asking for a cease-fire and an international presence to monitor Israel's commitment to it.