More than a month on, many people still need much more help

US Navy ships are due to leave Burma's coastline because of the continued refusal of the government to allow them to help victims of Cyclone Nargis.

The navy said it would withdraw the four ships, carrying helicopters and landing craft, after 15 failed attempts to convince the regime to let them in.

French and British navy ships have also been withdrawn after being refused permission to operate.

Cyclone Nargis left more than 133,000 people dead or missing.

More than a month after the disaster, the UN estimates that 2.4 million people are in need of food, shelter or medical care, and more than a million have yet to receive foreign aid.

'Saddened and frustrated'

Admiral Timothy Keating, commander of US Pacific Command, said the Burmese government had refused the navy's offer of help "each and every time".

"I am both saddened and frustrated to know that we have been in a position to help ease the suffering of hundreds of thousands of people and help mitigate further loss of life, but have been unable to do so because of the unrelenting position of the Burma military junta," he said.

Aid agencies say a huge operation is needed to help cyclone survivors

With aid agencies reporting continued difficulties in operating inside the affected delta region, correspondents say the 22 heavy-lift helicopters on board the US ships could have made a real difference.

"Important heavy-lifting capability in the delta would have been a standard operating procedure for relief agencies in the response," said Paul Risley, a spokesman for the UN World Food Programme.

Despite the promise of full co-operation given by the ruling generals to UN Secretary-General Ban Ki-moon during a recent visit, international relief agencies still say there is an urgent need for better access.

They say they are still chronically short of transport, and are being given only intermittent access to the delta.

Correspondents say it is proving hard to get a clear overall picture - but in its most recent report the UN said there was a "serious lack of sustained humanitarian assistance for the affected populations".

Ten commercial helicopters have now been chartered by the UN, but these are expensive and still insufficient for the task.