Israeli forces thrust deep into central Gaza on Thursday, killing a senior Hamas leader, in what may be a final push against the Islamist group before agreeing to a ceasefire in the 20-day-old war. Said Siam, was in his brother's house in a Gaza City suburb when an Israeli warplane dropped a huge bomb, flattening the building and leaving a deep crater in the sandy ground. One of Siam's sons, Siam's brother and the brother's family were also killed. Hamas TV showed footage of Siam's body, wrapped in a bloodied white shroud. Siam was the second of Hamas' top five to be assassinated in the Gaza conflict which started on Dec. 27. Diplomatic sources said Israeli leaders were studying Hamas's terms for a truce. They included a year-long, renewable ceasefire, the withdrawal of all Israel forces within 5 to 7 days, and the immediate opening of all Gaza border crossings, backed by international guarantees they would stay open. Israel originally said it would reject any agreement which had a predetermined ceasefire timetable over fears Hamas would use that time to rearm and regroup, which Israel alleges it did in the previous six month ceasefire. UN Secretary-General Ban Ki-moon, speaking after talks in Jerusalem with Israeli leaders, said “I understand today the Israeli government will make an important decision on a ceasefire. I hope that decision will be the right one.” “I am reasonably optimistic,” Ban told reporters later, but it might take “a few more days” to work out details. Ban said he discussed Gaza in telephone calls with Egyptian President Hosni Mubarak, who has been negotiating a truce with Israel and Hamas, as well as US Secretary of State Condoleezza Rice and British Prime Minister Gordon Brown. “(Mubarak) thought that they were going very close to a conclusion, to a ceasefire,” Ban said. “But there seem to be some other detailed technical issues which need to be further elaborated and agreed.” But he urged both Israel and Hamas to stop their fighting before any such technical details are worked out. “They first should stop the fighting, agree to an immediate ceasefire and continue their negotiations,” Ban said. The issue of how long the ceasefire would last, he said, did not seem to be a problem. More difficult were issues such as preventing the illegal smuggling of weapons into Gaza, he said. In a step to bring a deal closer, the United States told Israel it would be prepared to offer security guarantees. Meanwhile, Israeli Foreign Minister Tzipi Livni traveled to Washington to to conclude a US-Israeli agreement on arms smuggling into Gaza. “We are discussing with the Israelis and others what we can do to bolster the possibilities of getting to the durable ceasefire that we are all seeking,” Secretary of State Condoleezza Rice told reporters. “There are several elements to that and we are working with regional partners and also with the Israelis.” “We are aggressively working for that ceasefire and we are trying to help put the pieces in place so that it can be durable,” she said. EU blasts attack on UNRWA The EU presidency condemned the strike on a United Nations Relief and Works Agency (UNRWA) building in Gaza by Israeli artillery according to an official statement. “The (EU) Presidency demands that Israel undertake measures to prevent any recurrence of this attack on civilian or humanitarian targets, which is simply unacceptable,” it added. The statement called on both parties to “respect the principles of international humanitarian law” and to put an end to the suffering of the civilian population.”