Aid agencies have called for an urgent airdrop of 200,000 tents to Haiti
The acting head of the UN mission in Haiti has said reconstruction will take several decades, following the devastating earthquake two weeks ago.
Edmond Mulet told the BBC the logistics of the relief effort were a nightmare, with Haiti's inadequate infrastructure destroyed and a shortage of vehicles.
Mr Mulet said reconstruction was not starting at zero, but "below zero".
As many as 200,000 people died in the earthquake on 12 January, while an estimated 1.5 million are now homeless.
The UN has estimated that 75% of the capital, Port Au Prince, will have to be rebuilt. Salvage crews have begun to clear the rubble.
On Thursday, the UN held a memorial service in the Haitian capital, Port-au-Prince, to remember dozens of its staff who were killed.
People hugged each other as the list of 85 confirmed UN fatalities, including mission head Hedi Annabi, was read out. Dozens are missing.
At the service, a visibly emotional Mr Mulet read a message from UN Secretary General Ban Ki-moon: "To you I say: we are with you in spirit. To those no longer with us I say: we will never forget you."
Later, Mr Mulet told the BBC that all of Haiti's recent development had been undone.
"I think this is going to take many more decades than only 10 years and this is an enormous backwards step in Haiti's development. We will not have to start from zero but from below zero," he said.
The UN's assistant secretary-general for peacekeeping operations, who is acting head of mission in Haiti, described the logistics of the relief operation as a "nightmare", but said those involved were still managing to improve their capacity to provide the help needed.
"All this is coming together right now," he said. "Every day you can see more and more Haitian national police on the ground, working with our troops and more and more water being distributed, so it's a matter of time and putting all these elements together."
He said areas around Port-au-Prince were being prepared for the creation of tented settlements for the homeless currently living in makeshift camps.
Edmond Mulet said all of Haiti's recent development had been undone
The Inter-American Development Bank (IDB) was working with UN and Brazilian engineers at one site near Croix-des-Bouquets that would house around 30,000 people, and four other sites had been identified, he added.
Mr Mulet said 200,000 heavy-duty tents had been ordered to cope with Haiti's rainy season, which typically begins in May, and its hurricane season, which is expected to start around June.
"Of course, 200,000 family-sized tents - solid ones that can withstand a hurricane season - are not available in the market just like that, so they have to be made. It's going to take a few days and weeks before they can arrive, but all this is coming," he added.
Haitian President Rene Preval earlier this week called for the urgent airdrop of 200,000 more tents and 26 million ready-to-eat meals before the rainy season begins.