UN Secretary General Ban Ki-moon made the announcement as Burma's military leaders seized aid shipments destined for cyclone survivors and told the top US diplomat there they are not ready to allow American aid workers into the country.

Mr Ban said he has received reports from his staff that conversations had begun with Burmese officials to break the gridlock, although he said leaders of the military junta had "regrettably" not yet made direct contact with him.

"It's moving toward the right direction," he said.

Members of the Disasters Emergency Committee said they "could do more" without opposition from the military junta.

Tim Costello, chief executive of World Vision Australia, said suspicion of foreign aid workers was hampering their work.

He warned of an epidemic of "apocalyptic proportions" if aid does not get through.

But he urged donors to continue giving, saying the charities were making a difference.

The DEC appeal by British aid agencies and charities has raised £4m in two days.

Mr Costello said the Burmese government did not have the capacity to run the relief effort.

Speaking from Burma to a press conference in London, he said: "The frustration is we know we could do more.

"We know what the potential is but actually we are getting to people and making a difference."

He said the affected area contains 19 million people and around two thirds of them are children.

He went on: "We certainly know that once an epidemic starts it's difficult to stop and becomes of apocalyptic proportions.

"The potential for this epidemic is extremely probable."

He said the relief effort faced a "race against time" to prevent diseases such as diphtheria, cholera and malaria spreading.

"The size of this is simply extraordinary and in terms of its impact much greater than the tsunami impact in Sri Lanka or Indonesia.

"The capacity to absorb this level of suffering, dislocation and chaos doesn't exist (in Burma). It really requires international aid, it cannot be done in-country."

Yesterday, the UN World Food Programme announced it had halted aid after initial deliveries were seized by the Burmese regime.

The consignment included 38 tonnes of high-energy biscuits and relief equipment.

Between 60,000 and 100,000 people are believed to be dead or missing in the cyclone, the British Ambassador in Rangoon Mark Canning said yesterday.

The cyclone struck last Saturday flooding much of the Irrawaddy delta, leaving millions homeless.

Mr Costello said the Burmese government appeared concerned that aid workers were actually foreign observers reporting on today's planned constitutional referendum.

He said he hoped the suspicion attached to aid workers would relax after the vote.

And he rejected the call by Liberal Democrat leader Nick Clegg for aid to be parachuted into Burma against the wishes of the authorities.

"I think that would probably be counter-productive," he said.

"Metaphorically aid is like a tap dripping when we need a full fire hose to flow and reach those who are desperately in need."

British Red Cross director Matthias Schmale said the Burmese were being "slow" in approving visas for aid workers.

"Hopefully the door we do see opening will open further," he said.

Save the Children chief executive Jasmine Whitbread, urged people to carry on giving, saying the money is making a difference.

"Our message is that we are reaching these people and we desperately need the funds to keep flowing so our operations can continue."

She said it would be a tragedy if aid stopped flowing because people doubted that the aid was getting through.

The charities are trying to deliver food, water purification tablets, basic shelter, plastic sheets and medicine – ahead of fears that bad weather could return to the region next week just as the floodwaters begin to clear.