GAZA CITY (AFP) - The United Nations urged Israel on Thursday to reopen Gaza crossings as senior officials assessed war damage and the Jewish state warned it would strike again if Hamas rearms through smuggling tunnels. "If you want to have reconstruction, you have to have cement and construction materials and pipes and spare parts," said UN humanitarian chief John Holmes at a UN-run school hit by an Israeli missile in the northern town of Beit Lahiya. "Everything has got to come in; that is one of the things we will be insisting on strongly" in discussions with Israel, said Holmes who was touring Gaza along with UN Middle East envoy Robert Serry. Since the bombing began on December 27, the army said it had allowed 2,284 truckloads of humanitarian supplies into Gaza where the 1.5m population relies on the border crossings for virtually everything. "It is particularly saddening and sickening to see a school destroyed like this," said Holmes at the site of one of four UN-run schools hit by Israeli strikes during the 22-day war that left much of Gaza in ruins. Speaking a day after new US President Barack Obama called Israeli and Palestinian leaders, Serry said the sputtering Middle East peace process had to be renewed in order to avoid such wars. "I very much hope that very soon... with vigour a peace process will be renewed because the only reasonable way out is a two-state solution," he said. Hamas' chief has called on the West to lift a ban on contacts with his Islamist movement, which is pledged to the destruction of Israel and is banned as a terror outfit by the Jewish state, Washington and most of Europe. "Now it is time to start to talk to Hamas," which has controlled the Gaza Strip since June 2007, the exiled Khaled Meshaal said. A senior aide to Palestinian President Mahmud Abbas, whose forces were booted out of Gaza by Hamas, said that the Islamists will not be allowed to turn Gaza into a separate entity in the wake of the war. "We will not allow the creation of a separatist entity, no matter what the price," Yasser Abed Rabbo told a press conference. "The plot aiming to separate Gaza from the West Bank will not pass." Two women, two children and an elderly man were wounded on Thursday by fire from Israeli navy boats patrolling the Mediterranean, medics said. The army said it fired warning shots at a fishing boat. Otherwise mutual Israeli and Hamas ceasefires were holding for a fifth day. But Israel warned it would attack the territory again if Hamas uses smuggling tunnels under the Egyptian border to re-arm. "Things must be clear - Israel reserves the right to react militarily against the tunnels once and for all," Foreign Minister Tzipi Livni said. "If we are forced to, there will be more attacks," Barak said. During its offensive on Hamas, Israel bombed hundreds of tunnels on the Gaza-Egypt border, destroying 150, according to Defence Minister Ehud Barak whose warning echoed Livni's. As Israeli envoy Amos Gilad held talks in Cairo on consolidating the Gaza ceasefire, army radio reported Israeli officials had softened their position about releasing Palestinian prisoners in exchange for an Israeli conscript seized by Gaza activists in a cross-border raid in June 2006. The radio also said Prime Minister Ehud Olmert has decided to deploy maximum efforts in order to get Gilad Shalit, a Israeli solider Palestinians captured more than two years ago, back before Israel's February 10 elections.