DAVOS, Switzerland (CNN) -- United Nations Secretary-General Ban Ki-moon on Thursday launched a humanitarian appeal to provide emergency aid to the people of Gaza in the aftermath of Israel's military offensive in the region.
U.N. Secretary-General Ban Ki-moon warned that the cease-fire in Gaza was very fragile.
Ban, attending the World Economic Forum in Switzerland, said that $613 million was needed to re-establish basic services providing water, sanitation, healthcare and food to Gaza's 1.4 million people.
At least 1,300 Palestinians died and more than 5,000 were injured during the 22-day campaign, according to U.N. figures. Around 21,000 homes were destroyed or badly damaged. Thirteen Israelis were killed, 10 of them soldiers, during the conflict, according to Israeli authorities.
Israel says the military offensive was aimed at rooting out Hamas militants responsible for launching rocket attacks into the south of the country from Gazan territory.
Ban, who described the destruction in Gaza as "heartbreaking" during a tour of the territory last week, said the $613 million fund would be sufficient to meet the needs of the U.N. and other aid organizations in Gaza for the next nine months.
John Holmes, the head of the U.N.'s Office for the Coordination of Humanitarian Affairs (OCHA), also called on Israel to lift an 18-month blockade which had "deepened the vulnerability of Gaza's population," according to the U.N.
"We're asking for the crossing points to be opened fully -- otherwise, we won't be able to achieve what we want to achieve," Holmes told reporters.
A cease-fire between Israeli forces and Hamas militants has been in place since January 21.
But Israel launched a missile attack Wednesday, targeting tunnels along Gaza's southern border which it says are used by militants to smuggle weapons into the territory. That came after Palestinian militants detonated explosives in an attack on an Israeli military patrol on Tuesday, killing one soldier.
Ban said the cease-fire was "very fragile" and urged fresh diplomatic efforts to make it "more durable."
The near-four weeks of attacks destroyed public sector and private buildings in Gaza, affecting even the United Nations Relief and Works Agency's facilities and halting economic and social services, the Web site of the Palestinian Authority's Central Bureau of Statistics said.
Gaza's gross domestic product was slashed by 85 percent during the 22 days of war, and it could take a year for the economy to recover, the agency said in a preliminary report.
About 80 percent of crops in Gaza were destroyed, according to the agency.
Delegates at Davos are due to discuss the prospects of peace in Gaza and the Middle East at several sessions later Thursday.
Israeli President Shimon Peres, Iranian Foreign Minister Manouchehr Mottaki, Iraqi Foreign Minister Hoshyar Zebari and Tony Blair, Special Envoy to the Middle East for the U.N.-led Quartet, will join delegates.
Meanwhile Thursday one of the U.N. humanitarian agency's former employees launched a stinging attack on the agency for not doing enough to distance itself from Hamas militants, who are sometimes on the agency's payroll.
The accusation was leveled in a scathing report by James Lindsay, a former legal adviser for the U.N. Relief and Works Agency for Palestine Refugees in the Near East (UNRWA). He worked for the agency from 2000 until 2007.
He calls on the U.S. government -- one of UNRWA's largest donors -- to demand changes to the agency, which provides food and relief supplies to some 80 percent of Gaza's 1.5 million people.
Lindsay authored the 80-page report, released in January, for The Washington Institute for Near East Policy, a think tank that is widely viewed as a pro-Israeli lobbyist group. The institute, however, says that it seeks to provide "a balanced and realistic understanding of American interests in the Middle East."
Israel accused UNRWA of harboring Hamas militants in its shelters during the recent Gaza conflict, something the United Nations has vehemently denied. Gaza is controlled by Hamas, a Palestinian militant group that is considered a terrorist organization by the United States and Israel because of its refusal to renounce violence and recognize the Jewish state.
In his report, Lindsay said UNRWA "has taken very few steps to detect and eliminate terrorists from the ranks of its staff or its beneficiaries and no steps at all to prevent members of terrorist organizations such as Hamas from joining its staff."
He also accuses the agency of political bias and fiscal mismanagement.
UNRWA's chief spokesman, Chris Gunness, was quick to defend to the agency, saying that Lindsay's report "wholly misrepresents UNRWA and the services it provides to Palestine refugees."
He said the article demonstrated a "regrettable absence of balance and integrity" and "misled" readers by providing them "no access to pertinent (and) authentic information."
UNRWA plans to prepare a detailed response to the report.