Israel stepped up the pressure on Hamas on Thursday and dropped a one-ton bomb on the home of one of the group's top five leaders, Sheikh Nizar Rayyan, killing him and a reported 18 others as the IDF made final preparations for a ground operation expected in the coming days. Some security sources, however, said the likelihood of a massive IDF ground incursion into the Gaza Strip was receding. The IAF hit more than 40 targets throughout the day, with an emphasis on the homes of senior Hamas officials. Four homes were targeted, including the one in Jabalya that belonged to Rayyan, a senior Hamas commander and cleric for the terror group's military wing, Izzadin Kassam. More than 700 targets have been bombed since the beginning of the operation, and over 50 rockets pounded the South on Thursday, one of them striking a nine-story apartment building in Ashdod and sending 32 people into shock. Palestinian medics said Rayyan, his four wives and nine of his 12 children were among those killed in the strike. Another 30 people were wounded, and nearby buildings suffered extensive damage. The IDF said it had called the house before the strike and warned the family to leave. A large cloud of smoke billowed above the northern Gaza refugee camp after the attack. Rayyan's house, the IDF said, served as a weapons warehouse and as a Hamas communications center. In addition, a tunnel was reportedly located under the house and was used as an escape route for terror operatives. Security officials said Rayyan had served not only as the religious leader for Hamas's military wing, but also as one of its military commanders. He was often seen in uniform and participated in military exercises. He was said to have been close to Hamas terror commander Salah Shehadeh, who was killed by the IAF in 2002. Rayyan was both the director and the financier of the 2004 terror attack at the Ashdod port, which killed 10 Israelis, and in October 2001 he sent his son to perpetrate a suicide attack in the Gush Katif settlement Elei Sinai, where two Israelis were killed. Rayyan also reportedly replaced Sheikh Ahmed Yassin as the organization's top clerical authority after Yassin's assassination in 2004. In his last interview, recorded with Hamas TV on Wednesday, Rayan was as defiant as ever. "Oh fighters, know that you will be victorious," he said. "God promises us either victory or martyrdom. God is greater than they [Israelis] are, God is greater than their planes, God is greater than their rockets." Hamas spokesman Mushir al-Masri responded to the attack on Thursday afternoon, saying, "This is a new escalation in violence. We are taking every precaution to guard the Hamas leaders in order to ensure the enemy doesn't score further victories." "We are waiting for you to enter Gaza to kill you or make you into Schalits," the group said. IDF soldier St.-Sgt. Gilad Schalit was abducted in a cross-border raid by Hamas-affiliated terrorists two-and-a-half years ago, and remains in captivity in Gaza. Rayyan was the fourth senior terror figure targeted by the IAF on Thursday. Earlier in the day, the IAF bombed the home of Muhammad Barahoud, a top Popular Resistance Committees operative. The IDF said Barahoud was the head of all rocket cells in northern Gaza and that he was funded and supported by Hamas. The army said there were anti-tank missiles, rockets and bombs in the home. Another home destroyed belonged to Hasim Drili, a northern Gaza Hamas operative. The army said that he had a manufacturing plant in his home for rockets, mortar shells and missiles. The third home belonged to Tafik Abu Raf, a Hamas terror operative in the central Gaza Strip. He had a weapons laboratory in his house, according to the IDF. The fourth home belonged to Nabil Amrin, another senior terrorist and the commander of a Hamas battalion. The IDF reported a series of secondary explosions following the air strike, confirming that Amrin's home in the Sheikh Raduan neighborhood of Gaza City contained a large weapons and ammunition cache. While operations continued from the air on Thursday, defense officials said that all preparations for a ground operation had been completed and that a ground offensive would likely be launched in the coming days. The officials said the IDF had received the green light from the cabinet to launch a ground offensive, which they said was being held up by "operational considerations." "The ground offensive is ready," a top IDF officer said Thursday. The assessment in the IDF is that Israel has a narrow window of opportunity to launch a ground operation before the Western holiday season comes to an end and a surge of diplomatic activity is expected in the region. Officials said Israel would not conquer the Gaza Strip in the ground operation. On Thursday night, Barak decided to impose a total closure on the West Bank, starting on Friday and ending Saturday night. AP contributed to this report.