Survivors are beginning to gather in makeshift camps around the edges of the disaster zone caused by Cyclone Nargis eight days ago.
The UN, which has launched a $187m (£96m) appeal for aid, says survivors in the worst-affected areas urgently need food, shelter and medical aid.
Aid has been flown in from Burma's neighbours, such as China and Thailand, and the first US relief flight is expected to arrive on Monday.
The UN estimates that only a quarter of survivors have received any aid so far.
UK Foreign Minister David Miliband said the Burmese military government had made the tragedy worse.
"A natural disaster is turning into a humanitarian catastrophe of genuinely epic proportions in significant part because of what I would describe as the malign neglect of the regime," he told the BBC.
His comments came as fresh video footage emerged of the extent of the suffering in the wake of the disaster, with the corpses of children shown lined up in a makeshift morgue.
Warning of danger of 'humanitarian catastrophe' in Burma
But Andrew Kirkwood, the head of the UK aid organisation Save the Children in Burma, said there were some encouraging signs.
"It seems that material is being cleared through the airport much more easily," he told the BBC.
"People have been able to get hold of at least a minimum supply of boats and trucks and fuel. I think that's really helped."
The UN has also welcomed some "reasonably positive" signs from the Burmese authorities.
But the relief effort suffered a further blow as a cargo boat carrying aid including drinking water, rice and purification tablets to more than 1,000 survivors hit a submerged tree trunk and sank in the Irrawaddy Delta, the International Federation of the Red Cross said.
"The crew steered to an island but the boat sank rapidly," an IFRC statement said.
"All crew members, including four Myanmar Red Cross relief workers on board... managed to get to safety."
An IFRC official said the agency would now have to re-evaluate its transportation of aid.
The BBC's Andrew Harding in Bangkok says many survivors are reported to be badly dehydrated, the children in particular sick with diarrhoea.
After more than a week in the open, wounds are becoming infected and precious little aid is reaching most of these people, he says.
The International Rescue Committee said that without a massive delivery of aid, Burma faced "unimaginable tragedy".
"Unless there is a massive and fast infusion of aid, experts and supplies into the hardest-hit areas, there's going to be a tragedy on an unimaginable scale," said the US-based IRC's Greg Beck.
Agencies warn the death toll could rise to 1.5m unless aid is delivered
On Saturday, a convoy from the UN refugee agency, UNHCR, crossed into Burma from Thailand with 22 tonnes of tents and other humanitarian supplies.
Since the cyclone struck on 3 May, aid agencies already in the country have started relief efforts with supplies they had available and by buying from local sources.
Hospitals, schools and other large buildings are crammed with the displaced.
Christian Aid says people are asking for rice seeds, as their supplies have been damaged. Unless people plant the seeds in the next month, they will have no supplies of rice until May 2009.
Meanwhile, Burmese state television says turnout in Saturday's constitutional referendum was massive.
The vote went ahead in most of the country, despite international calls for the government to direct all its attention towards relief work.
The government says the referendum will pave the way for democracy, but the opposition says it was designed to cement the military stranglehold on power.
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