Aid started to trickle into devastated Burma last night as the UN estimated that Cyclone Nargis has affected 1.5 million people with 100,000 dead.

The UN's human rights chief Sir John Holmes said the situation was growing "increasingly desperate" with the country's military rulers only allowing delivery of a small portion of the international aid required.

The British Foreign and Commonwealth Office is "urgently" trying to trace 12 Britons believed to have been caught up in the killer storm.

Fears are growing for survivors, who are facing hunger and disease in the wake of the Cyclone Nargis which has wiped out a large part of the Irrawaddy delta region, the main rice growing area, and wrecked the infrastructure of the capital Rangoon.

The World Health Organisation (WHO) has received reports of malaria outbreaks in the Irrawaddy area, and said waterborne illnesses from dirty water and poor sanitation also remained a concern.

Sir John and UN 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 to concentrate on the tragedy.

Sir John, who is also UN Emergency Relief Co-ordinator, said that while assistance has started to arrive, the effort is clearly not adequate.

"We are faced here with a major catastrophe. The humanitarian situation is increasingly desperate on the ground, in the delta, because of the conditions that are obtained there. The number of people that have been affected - we estimate that number at about 1.5 million people severely affected."

He said there was a danger that a worse tragedy may unfold if aid does not arrive.

Officials were pressing for speedy access but he warned that any "confrontation" with the military government would not help those in need of help.

While four relief flights from the World Food Programme landed in Rangoon yesterday, progress had been limited and "frustrations have been growing," he said.

Burma was still not allowing US military planes to fly in critical supplies last night. It has been reported that two aircraft, loaded with high energy biscuits, water supplies and medicines, were refused entry. State media said Cyclone Nargis killed at least 22,980 people and left 42,119 missing, mostly in the hardest-hit Irrawaddy delta, but the US said the toll could be more than 100,000.

Entire villages in the delta were submerged, and bloated corpses could be seen stuck in the mangroves. Some survivors stripped clothes off the dead.

Even near Rangoon, stricken villagers said they had received no government assistance and were relying on Buddhist monasteries, which have been helping the public cope.

A UN spokesman said 30 to 40 visas requested by various groups were pending.

"These are mostly people who have key experience in handling disasters, and so they can bring lessons from other similar disasters. The agencies are becoming frustrated."

Amnesty International said donors were delaying aid for fear it would be diverted. 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. We are going to bring in not just supplies but capacity to go with them to make sure the supplies get to the people."

The UN has recorded donations to Burma relief of around £12m from 28 nations, the EU and charities, which includes £5m from the UK. An additional £12m has been pledged.

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