Gaza City Gaza Strip ? An Israeli warplane dropped a 2,000-pound bomb on the home of one of Hamas’ top five decision-makers Thursday, instantly killing him and 18 others, 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 attacks continued Friday. 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 people were hurt in the attacks, hospital officials said.

Even as it pursued its bombing campaign, 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.

Adding to the urgency, the Israeli military said its preparations for a possible ground assault were complete and that troops stood ready to cross the border if the air operation to stamp out Hamas rocket fire needed to be expanded.

Soldiers massed along the Gaza frontier said they were eager to join the fight, and some even cheered as they heard thunderous airstrikes in the distance.

The hit on Rayan’s home obliterated the four-story apartment building and peeled off the walls of others around it in the town of Jebaliya in the northern Gaza Strip.

Eighteen other people, including all four of Rayan’s wives and nine of his 12 children, also were killed, Palestinian health officials said.

The house was one of five bombed Thursday, among more than 20 targets altogether. Warplanes shredded the houses, taking off walls and roofs and leaving behind eerie, dollhouse-like views into rooms that still contained furniture.

Israel’s military, which has said the homes of Hamas leaders are being used to store missiles and other weapons, said the attack on Rayan’s house triggered secondary explosions from the arms stockpiled there.

Seven other Palestinians were killed in airstrikes Thursday, and one died of earlier injuries.

Israel has targeted Hamas leaders many times in the past, and its leadership went into hiding at the start of the offensive. Rayan, however, was known for openly defying Israel and in the past had led crowds to the homes of wanted Hamas figures — as if daring Israel to strike and risk the lives of civilians.