Up to 1.5 million people have been severely hit and as many as 100,000 killed by the Burmese cyclone, it emerged tonight.
The UN's human rights chief John Holmes said the situation was growing "increasingly desperate" with the country's military rulers still allowing in only a trickle of international aid.
The government appealed for international assistance after the storm struck on Saturday but have since dragged their feet on issuing visas to relief workers even as survivors faced hunger, disease and flooding.
Holmes and Secretary-General Ban Ki moon urged Burma to open its doors to the help available and to cancel plans for a referendum on a new constitution this weekend and concentrate on dealing with the tragedy.
Holmes, who is also UN emergency relief co-ordinator, said that while assistance has started to arrive, the effort is clearly not adequate given the enormity of the situation.
"We are faced here with a major catastrophe," he said.
He said there was a real danger that an even worse tragedy may unfold if desperately needed aid does not arrive quickly.
Holmes said officials were pressing for speedy access but he warned that any "confrontation" with the military government would not help those desperately in need of help.
"The humanitarian situation as you know is increasingly desperate on the ground, in the delta, because of the conditions that are obtained there," he said.
"The number of people that have been affected - we estimate that number at about 1.5 million people severely affected by the disaster."
While four relief flights from the World Food Programme landed in Rangoon today, progress had been limited and "the frustrations have been growing," he said.
Burma was still not allowing US military planes to fly in critical supplies.
State media said Cyclone Nargis killed at least 22,980 people and left 42,119 missing, mostly in the hardest-hit Irrawaddy delta. A US diplomat said yesterday the toll could be more than 100,000, however.
Entire villages in the delta were still submerged from the storm, and bloated corpses could be seen stuck in the mangroves. Some survivors stripped clothes off the dead.
Even near Rangoon, stricken villagers complained that they had received no government assistance and were relying on Buddhist monasteries, which have been helping the public cope with the disaster.
A UN spokesman in Bangkok said between 30 and 40 visas requested by various agencies and private relief groups were pending.
"These are mostly people who have key experience in handling disasters of this scale, and so they can bring lessons from other similar disasters," he said. "The agencies are becoming frustrated."
The London-based human rights group Amnesty International said some donors were delaying aid for fear it would be diverted to the army.
The World Food Programme's regional director Anthony Banbury indicated the United Nations had similar concerns.
"We will not just bring our supplies to an airport, dump it and take off," he said. "This is one reason why there is a hold up now, because we are going to bring in not just supplies but a lot of capacity to go with them to make sure the supplies get to the people."
Meanwhile Burmese state radio said "unscrupulous elements" were spreading rumours of an impending earthquake, a second cyclone and looting in Rangoon.
Residents say some looting occurred at markets and stores in suburbs earlier this week.
The warning about rumours appeared to be an attempt to calm the population as well as stop any gatherings that might turn into political agitation against widely detested military rule.