New video shows the force of the cyclone which ripped through Burma
International agencies are pushing to gain access for a massive aid operation in Burma, where the toll from Saturday's cyclone continues to rise.
Up to 15,000 people are said to have died in the disaster, and many more are missing, officials say.
Hundreds of thousands of people are said to be without clean water and shelter, with some areas still cut off.
Burma's leaders say they will accept external help, in a move correspondents
say reflects the scale of the disaster.
The military junta has said it will postpone to 24 May a referendum on a new constitution in areas worst-hit by the cyclone - including the former capital Rangoon and the Irrawaddy Delta, state television said on Tuesday.
But the vote initially planned for 10 May will proceed as planned in the rest of the country, the report said.
"Looking at the number of deaths, it leads us to think that an early warning system had not been put in place," Brigitte Leoni, spokeswoman for the UN International Strategy for Disaster Reduction, told journalists in Geneva.
"Obviously many people did not have time to evacuate and find refuge in secured buildings," she said.
Earlier, US First Lady Laura Bush, who takes a special interest in Burma, accused the Burmese authorities of failing to give a "timely warning" about the approaching storm.
Work is still under way to assess the scale of the devastation caused by Cyclone Nargis, which brought winds reaching 190km/h (120mph).
More deaths were caused by the tidal wave than the cyclone itself, Minister for Relief and Resettlement Maung Maung Swe told a news conference in Rangoon on Tuesday.
"The wave was up to 12ft (3.5m) high and it swept away and inundated half the houses in low-lying villages," he said. "They did not have anywhere to flee."
Some 95% of the homes in the city of Bogalay in the Irrawaddy river delta were destroyed, he added.
The director for the World Food Programme in Burma, Chris Kaye, said information about the destruction in the Irrawady Delta was still emerging, but it was clear it was on a very large scale.
"We have a major humanitarian catastrophe in our hands. The numbers of people in need are still to be determined, but I'm sure we're talking of hundreds and thousands," he told the BBC World Service's World Today programme from Rangoon.
Andrew Kirkwood, Burma country director for Save the Children, said there were positive signs from the Burmese authorities, who have traditionally been suspicious of aid agencies, limiting their activities.
"Every indication is that everyone realises that this is an unprecedented event in Myanmar's [Burma's] history and the government is much more open to international assistance than it has ever been."
DEADLIEST RECENT STORMS
Hurricane Katrina, US, 2005 - at least 1,836 dead
Orissa Cyclone, 1999, Northern India - at least 10,000 dead
Hurricane Mitch, 1998, Central America - at least 11,000 dead
In the low-lying Irrawaddy Delta region, the storm caused a sea surge that smashed through towns and villages.
"Those areas in the southern part of the Delta - Bogalay, Laputto and Pyapon - were very severely affected, particularly by the storm surge," Mr Kaye told the BBC.
"And a storm surge in a low-lying area such as that, coupled with very high winds, clearly has served to flatten large areas of that part of the delta, and of course taken villages and villagers with it."
The storm destroyed roads, downed power lines and flattened houses, leaving people across the region homeless.
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