Many still need help after Burma's worst ever disaster
Relief and reconstruction work in Burma after Cyclone Nargis will cost at least $1bn (£500m), according to the UN and the regional body Asean.
The figure is in a report released at Asean's annual meeting in Singapore.
It is the first comprehensive assessment of the damage caused by the cyclone on 3-4 May, which is believed to have killed 130,000 people.
Burma's ruling generals were criticised in the wake of the cyclone for being slow to accept international aid.
Asean has already played a key part in helping to facilitate exchanges between Burma's ruling junta and international donors.
Asean Secretary-General Surin Pitsuwan told a news conference that the three parties involved in the report - the UN, Asean and the Burmese government - needed at least $1bn to deal with "a tragedy of immense proportions".
The estimated figure covers the most urgent needs such as food, agriculture and housing for the next three years.
"The task ahead is clearly enormous and will take a lot of time, a lot of effort," Mr Surin said.
"While significant progress has been made to date, we are still in the relief phase for this aid operation," added the UN humanitarian chief John Holmes.
The report outlines the scale of the cyclone - Burma's worst ever disaster - and estimates that it destroyed 450,000 homes, damaged 350,000 others, flooded 600,000 hectares of agricultural land and destroyed 60% of farming implements.
About 75% of hospitals and clinics in the area were destroyed or badly damaged.
Burma's military rulers are under the spotlight as delegates convene at the Asean meeting.
On Sunday, delegates issued a rare statement criticising the isolated nation, urging it to release political prisoners.
Asean is normally reluctant to criticise member states
They expressed "deep disappointment" over the junta's one-year extension of opposition leader Aung San Suu Kyi's detention.
In the past, the bloc has been accused of being too reluctant to speak out about the internal affairs of its member states.
The other issue on the agenda at the Asean meeting on Monday was the escalating tension between two other member states - Thailand and Cambodia - over ownership of the area around the ancient temples of Preah Vihear.
"The situation has escalated dangerously, with troops from both sides faced off on disputed territory near the Preah Vihear temple," Singapore Prime Minister Lee Hsien Loong told delegates in his opening speech.
He added that he had received assurances from both countries that they would exercise "utmost restraint" and abide by international laws to resolve the issue amicably.