As more than 50 mortar shells, Kassam and Katyusha rockets were fired into Israel from Gaza on Thursday, the Defense Ministry was still working to "exhaust" the dialogue with Egypt on a cease-fire with Hamas, sources in the Prime Minister's Office said. Amos Gilad, head of the Defense Ministry's Diplomatic-Security Bureau, returned Thursday night from a meeting in Cairo with Egyptian Intelligence chief Omar Suleiman to try to finalize a deal. Defense officials said Gilad asked the Egyptians for clarifications on the release of kidnapped soldier Gilad Schalit in conjunction with a truce deal, as well as on Cairo's promise to step up efforts to curb the smuggling of weapons into Gaza under the Philadelphi Corridor. "The Egyptians are supposed to get back to us with answers in the coming days," a senior defense official said. "It is likely that the cease-fire will go into effect by the middle of next week." The barrage of rockets on the western Negev on Thursday was interpreted by the defense establishment as an attempt by Hamas to flex its muscles before implementing a cease-fire. There are fears that Hamas will try to carry out a massive terrorist attack in the coming days. "They want to have the last word before the cease-fire," the senior defense official said. "But if they continue with their attacks we will not sit by idly and will respond harshly as well." Prime Minister Ehud Olmert's spokesman, Mark Regev, said Thursday's attacks "show the current situation is both unstable and not sustainable, and that there has to be a solution." Regev said that the fact Hamas fired dozens of mortars the day after Israel declared it wanted to give the Egyptian cease-fire track an additional chance "shows who we are up against. This shows who Hamas is. They are committed to violence, not dialogue." Still, he said, Israel would give the Egyptian-mediated talks a chance, while at the same time continuing to plan for military action "in the unfortunate event that they won't be successful. The situation in the South is expected to be one of the key items on the agenda of talks US Secretary of State Condoleezza Rice will hold in Jerusalem on Sunday and Monday. Diplomatic officials say the US has been urging Jerusalem to let the Egyptians try to put together a cease-fire, concerned that an IDF invasion of the Gaza Strip could end the current diplomatic process with the Palestinians and badly damage Palestinian Authority President Mahmoud Abbas's stature. Rice, who is scheduled to arrive Saturday night and leave on Monday, is expected to hold two different sets of trilateral talks here to discuss the ongoing Israeli-PA negotiations. One of those trilateral meetings will be with Foreign Minister Tzipi Livni, the head of Israel's negotiating team, and her PA counterpart, Ahmed Qurei, to discuss the status of those talks, and the second trilateral meeting will be with Defense Minister Ehud Barak and PA Prime Minister Salaam Fayad to discuss implementation of road-map obligations.