JERUSALEM — Israel broadened the scope of its air offensive against the Hamas infrastructure in Gaza on Thursday, destroying important symbols of the government and, for the first time in its 6-day-old campaign, killing a senior leader of the militant Islamic group.
An Israeli warplane dropped a 2,000-pound bomb on the home of one of Hamas' top five decisionmakers, instantly killing him and 18 others, including his four wives and his children, while the Israeli army said troops massed on the Gaza border were ready for any order to invade.
The airstrike on Nizar Rayan was the first that succeeded in killing a member of Hamas' highest echelon since Israel began its offensive Saturday. The 49-year-old professor of Islamic law was known for personally participating in clashes with Israeli forces and for sending one of his sons on a 2001 suicide mission that killed two Israelis.
The Israeli military said there were many secondary explosions after the air attack that killed Rayan, "proving that the house was used for storing weaponry." It was also used as a communications center, the statement said, and a tunnel that had been dug under the house was used by Hamas operatives.
Before dawn, Israeli aircraft hit 15 houses belonging to Hamas militants, Palestinians said. They said the Israelis either warned nearby residents by phone or fired a warning missile to reduce civilian casualties. Twelve were hurt in the attacks, hospital officials said.
Earlier on Thursday, Israeli warplanes and naval forces bombed Hamas security installations, tunnels used for smuggling weapons, as well as symbols of the government like the Parliament building — a Gaza landmark — and the Ministry of Justice, the Israeli military said.
Even as it attacked, Israel kept the way open for intense efforts by leaders in the Middle East and Europe to arrange a cease-fire. Israel said it would consider a halt to fighting if international monitors were brought in to track compliance with any truce.
More than 400 Gazans have been killed and some 1,700 wounded since Israel embarked on its aerial campaign, Gaza health officials said. The United Nations has said the death toll includes more than 60 civilians, 34 of them children.
Israel launched the offensive Saturday after more than a week of intense Palestinian rocket fire that followed the expiration of a six-month truce, which Hamas refused to extend because Israel kept up its blockade of Gaza.
Israel's bruising campaign has not deterred Hamas from assaulting Israel. According to the military, militants fired more than 30 rockets into southern Israel during the day.
No injuries were reported, but an eight-story apartment building in Ashdod, 23 miles from Gaza, was hit.
Foreign Minister Tzipi Livni, speaking to reporters during a visit to Paris for meetings with French officials, expressed skepticism about the benefits of a cease-fire. She said Hamas used the lull during the six-month truce that expired last month to build up its arsenal of weapons.
"Our experience from the past is that even when we accept something in order to have a peaceful period of time, they abuse it in order to get stronger and to attack Israel later on," Livni said.
Gaza has been under Hamas rule since the group's fighters 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 truce monitors would require cooperation between the fiercely antagonistic Palestinian factions.
With Israeli troops and tanks massing along the border with Gaza in preparation for a possible ground invasion, Israel also pursued diplomatic avenues to explain its positions.
Most Hamas leaders in Gaza have been in hiding since the Israeli operation began, but Rayan was said to have refused to leave his home on ideological grounds. In the past, he had been known to gather supporters to stand on the rooftops of other houses in Gaza that Israel had threatened to strike.
Information from the Associated Press and the New York Times was used in this report.