International relief organisations were yesterday racing to test an agreement by the Burmese military junta to open the door to foreign aid workers, three weeks after Cyclone Nargis devastated the country.

Ban Ki-moon, the UN Secretary General, who will be in Burma today for a donors' conference, said yesterday that the country's ruling generals had told him that international aid workers, "regardless of nationality", will be able "to freely reach the needy people", a pledge the junta has not publicly acknowledged. But the UN World Food Programme (WFP) said the real test was whether the secretive regime allowed aid experts free access to the Irrawaddy delta, the area that suffered the full force of the cyclone on 2 May.

The Burmese authorities estimate the death toll at about 78,000, with another 56,000 missing. Aid organisations have been warning that without an immediate response, 2.5 million more people could be at risk from disease, hunger and exposure after losing their homes, but they estimate that only a quarter have been reached so far. The relief effort has encountered constant obstruction, with the regime at first seizing aid shipments and insisting on distributing the supplies itself. One general was shown on television handing a victim a box of food which displayed his name prominently, obscuring a smaller sticker which read: "A gift from the Kingdom of Thailand".

Although foreign aid officials were later grudgingly given visas and were able to take delivery of shipments arriving at Rangoon, the main city, they were kept out of the worst-hit areas of the delta, with only local staff allowed to take in relief supplies.

"We're hopeful that it [the pledge] means more foreign aid workers will go to the worst-affected areas," said Kate Conradt of Save the Children. "We already have a number of expatriate staff [inRangoon]. They just can't leave the city."

Other aid workers said they would be unable to mount a full-scale relief operation until they had carried out a proper assessment in all the areas affected by the disaster.

"We could have been at full stretch three days after the cyclone," said one official, who asked not to be named for fear of antagonising the junta. "Instead it is going to be more than three weeks."

The EU aid chief, Louis Michel, said: "We have no more time to lose, so it's imperative that the ... authorities immediately provide the international community with the practical details of the agreement."

The WFP, which co-ordinates emergency logistics for all UN agencies and many non-governmental organisations, set up a staging area at Bangkok yesterday for relief flights to Rangoon. It has also been given permission to bring in 10 helicopters to deliver supplies, and expects to have all of them in position by the end of this week. But on Friday, the head of the junta, Than Shwe, refused to allow British, American and French naval vessels, which have been waiting offshore for more than a week with relief supplies, to come into port, bringing criticism from France's President Nicolas Sarkozy, who said the generals had "once again made the wrong decision".

According to a UN official who accompanied Mr Ban to his meeting on Friday there were indications of disagreement within the junta. The official said that when his aides suggested that maybe too many concessions were being made, the 75-year-old general butted in: "I don't see a problem." But in a sign that the generals remain detached from the disaster, the regime carried on yesterday with voting in Rangoon and the Irrawaddy delta on a new constitution which entrenches their hold on power. A fortnight ago, their only concession to the worst natural calamity in Burma's history was to delay the referendum in those areas while it proceeded in the rest of the country.

The UN has launched an emergency appeal for just over $200m (£101m), but less than half that sum has been received in donations and pledges. Although the figure could be expected to increase if surveys can be made in the Irrawaddy delta, it also reflects scepticism about how much supervision the international community will be allowed to exercise on the ground. Ahead of today's donor conference, the Burmese authorities have estimated the economic damage caused by the cyclone at about $11bn.

Frustration at the delays in admitting aid led several figures, including the French Foreign Minister, Bernard Kouchner, and the Conservative leader, David Cameron, to call for supplies to be dropped to the survivors without the approval of the junta.

Gordon Brown, who called the regime "inhuman" for denying access to foreign aid workers, did not rule out the idea, but it attracted little support elsewhere. The main fear was that the junta would immediately halt the limited amount of international aid it was then allowing into the country, cancelling out any benefit to the victims. But some aid experts also believed the idea was impractical. Pierre Carrasse, chief logistics officer of the UN World Food Programme, disputed a claim by Karel Vervoort, a former major general in the Belgian air force, that survival packs could have been dropped to survivors in the disaster zone, using a system called Snowdrop.

Gen Vervoort, who helped to develop the system, wrote in The Independent on Sunday last week that thousands of individual parcels, weighing up to 5kg (11lb) each and containing enough supplies to keep one person alive for one day, could have been dropped within 24 hours of the cyclone, given food supplies and political will. But Mr Carrasse said the system was not suitable for a flooding disaster, and that 80 per cent of the supplies would have been lost. Splitting aid consignments into thousands of individual parcels was also time-consuming and expensive.

The logistics chief said that the WFP had helped to finance development of the Snowdrop technique, over which there is now a patent dispute. But in the agency's view it was only suitable in certain specialised situations, mainly when a population was displaced and on the move, and not for natural disasters.

Interesting? Click here to explore further