UN chief demands investigation into Israeli attack on Gaza headquarters • Ban Ki-Moon visibly angry after seeing burned-out UN buildings • Palestinians put property damage from conflict at £1.3bn Owen Bowcott guardian.co.uk, Tuesday 20 January 2009 14.43 GMT The United Nations secretary general today called for an investigation into Israeli attacks on UN buildings in the Gaza Strip as he saw for himself the damage caused by the three-week assault on the Palestinian territory. Ban Ki-Moon, visibly angry as he attended a ceremony at the burned-out UN headquarters in Gaza, described the attacks as "outrageous", demanding a thorough legal investigation and guarantees that such attacks would not be repeated. During the 22-day offensive, Israeli shelling hit the UN headquarters in Gaza as well as two UN schools. The attacks caused serious damage to the headquarters and killed nearly 40 people near one of the schools. "It has been especially troubling and heartbreaking for me as secretary general that I couldn't end this faster," he said. He warned that the ceasefire agreed on Sunday was fragile, and called on Israel and Hamas to "exercise maximum restraint and nurture the ceasefire". Thousands of Hamas supporters gathered for what the Islamist group's leaders described as a victory rally in a square outside the remains of the parliament building in Gaza City, which was heavily damaged in an Israeli air strike. Two men hoisted a sign in Hebrew reading: "The resistance will be victorious, Israel has been defeated". Ban is the first international statesman to tour Gaza since Israel launched its intensive air and ground assault on 27 December. Israel is expected to complete its troop pullout this afternoon, removing them from Gaza before Barack Obama is sworn in as US president. The Israeli government is eager to avoid political complications with its closest ally on inauguration day. Israeli sources said Ban would visit southern Israel, which has been the target of Hamas rocket attacks, during his Middle East peacemaking mission. Financial estimates of the full cost of the Israeli bombardment of Gaza reflect the scale of the destruction: the Palestinian Central Bureau of Statistics put total property damage at $1.9bn (£1.3bn). The UN's humanitarian chief, John Holmes, speaking in New York, said the organisation's staff in Gaza were trying "to find out as much as they can about how great the damage is and how great the needs are". He said: "On the purely humanitarian and early recovery side ... it will be hundreds of million of dollars and no doubt the overall reconstruction costs will be numbered in billions of dollars, but I wouldn't want to put a figure on it beyond that." Saudi Arabia has already pledged $1bn towards repairing the damage. European Union foreign ministers are due to meet in Brussels shortly to discuss humanitarian aid and Israeli demands for the prevention of weapons smuggling into Gaza. More than 1,300 Palestinians have been killed in Israel's attacks, which Israel said were intended to halt cross-border rocket attacks and destroy Hamas's military infrastructure. Hamas has said 5,000 homes, 16 government buildings and 20 mosques were destroyed and 20,000 houses were damaged in the assault; Israel claimed militants hid weapons inside the mosques.Palestinian militant groups said 112 of their fighters and 180 Hamas policemen were killed. Israel put its dead at 10 soldiers and said three civilians were killed in rocket attacks. Medical officials have confirmed that 700 Palestinian civilians were killed, and that figure is likely to rise as survivors return to search through the rubble of their shattered homes. As many as 400,000 Gazans have been left without running water. Israel, which accused Hamas of endangering non-combatants by operating in densely populated areas, alleged that hundreds of militant Hamas fighters were among the dead. In Geneva, the director general of the World Health Organisation, Margaret Chan, warned of a looming health crisis among the 1.5 million Gazans. She said she was "deeply concerned" about the interruption of immunisations and other life-saving care, and the limited availability of hospital beds in Gaza. Last night an Israeli man was shot and seriously injured while driving near the Kochav Hashachar settlement in the West Bank. The previously unheard of al-Bashair army claimed responsibility for the attack.