A tropical depression swirling southwest of Burma's main city Rangoon could develop into a cyclone within the next 24 hours, US experts say.

"The potential for the development of a significant tropical cyclone within the next 24 hours is upgraded to good with the only limitation being temporary land interaction," said a report by the Joint Typhoon Warning Centre, a website used by US government agencies.

The JTWC website said the depression was currently 30 nautical miles west-southwest of Rangoon.

Earlier the United Nations issued a similar warning after the JTWC alert.

Amanda Pitt, spokeswoman for the United Nations humanitarian relief program, told reporters another cyclone was likely, saying: "This is terrible."

Burma is still struggling to recover from Cyclone Nargis, which roared through its rice bowl on May 3, killing up to 100,000 people and leaving 1.5 million people destitute.

Heavy rains have been pelting homeless cyclone survivors in Burma's Irrawaddy delta, complicating the already slow delivery of aid to victims facing hunger and disease.

Critics are ratcheting up the pressure on its military rulers to accelerate a relief effort that is only delivering an estimated tenth of the supplies needed in the devastated delta.

"The response of the regime in Burma to this crisis has been absolutely callous and those paying the price of this callousness have been the long-suffering Burmese people," Australian Prime Minister Kevin Rudd told parliament on Tuesday.

An Australian air force plane landed in Rangoon, Burma's main city, with 31 tonnes of emergency supplies, a day after the first US military aid flight arrived in a country that Washington has described as an "outpost of tyranny".

Two more US flights arrived on Tuesday as part of a "confidence building" effort to prod Burma's reclusive generals into allowing a larger international relief operation 11 days after the disaster left up to 100,000 dead or missing.

France, Britain and Germany called for the world to deliver aid without the junta's agreement, using a little used UN principle of the "responsibility to protect".

The United Nations is worried that some of the aid intended for victims might have been diverted but has no hard proof of this, a UN spokeswoman says.

Speaking at a regular news conference, spokeswoman Michel Montas was asked if the UN was concerned that some of the aid sent to Burma, might be going to people who were not victims of Cyclone Nargis.

"That concern exists," she said on Tuesday.

"We don't have any independent report of a specific portion of the aid going to other sectors besides the victims (but) it is a fact that a very small percentage of victims have so far received the aid."

Burma state television said the official death toll had risen to 34,273 from nearly 32,000 and 27,838 were missing.

Tens of thousands of people throughout the delta are crammed into Buddhist monasteries and schools after arriving in towns that were on the breadline even before the disaster.

Lacking food, water and sanitation, they face the threat of killer diseases such as cholera. Heavy tropical rains added to their misery.

"Where I am now there's over 10,000 homeless people and it's pouring rain," Bridget Gardener of the International Red Cross said during a rare tour of the delta by a foreign aid official.

While a steady stream of aid flights have landed in Rangoon, only a fraction of the relief needed is getting to the delta due to flooding and the junta's desire to keep most foreign aid and logistics experts either out of the country or in Rangoon.

The World Food Program said it was able to deliver less than 20 per cent of the 375 tonnes of food a day it wanted to move into the flooded delta.

Burma state television said six ships carrying 500 tonnes of supplies had left Rangoon for the delta on Tuesday.

International relief organisations say their local staff are stretched to breaking point, while Medicins Sans Frontieres said its workers faced "increasing constraints".

One Rangoon businessman just back from a personal aid mission to Bogalay, a delta township where at least 10,000 people were killed, said soldiers were appropriating aid.

"Around Bogalay, private donors are not allowed to distribute their assistance to the victims themselves. We had to hand over what we had," he said.

The junta has welcomed "aid from any nation" but has made it very clear it does not want outsiders distributing it.

At the United Nations in New York, Secretary-General Ban Ki-moon delivered his most critical comments to date.

"I want to register my deep concern - and immense frustration - at the unacceptably slow response to this grave humanitarian crisis," he told reporters on Monday.

"We are at a critical point," he said. "Unless more aid gets into the country very quickly, we face an outbreak of infectious diseases that could dwarf today's crisis."

With three US and one French warship laden with aid and helicopters steaming towards Burma, EU foreign policy chief Javier Solana again raised the question of unauthorised aid drops into the delta - which could be seen as an act of war.

"We have to use all the means to help those people," he said.

The storm raged through an area home to nearly half of Burma's 53 million people. About 5,000 square kilometres of land remain under water.

Meanwhile, Tibetan spiritual leader the Dalai Lama laments that the killer cyclone in Burma will only deepen poverty in one of the planet's poorest countries.

"I am deeply saddened by the catastrophe caused in Burma by the recent Cyclone Nargis," the Tibetan Buddhist spiritual leader said in a letter to Jacob Kellenberger, president of the International Committee of the Red Cross.

"I am shocked by the destruction, especially by the high number of people who have perished and as a result this catastrophe has compounded the problems of poverty that already exists in Burma," he said.

The 72-year-old Dalai Lama has previously refrained from commenting on largely-Buddhist Burma, which is ruled by the military and borders India, analysts said.

The Nobel laureate made the northern Indian hilltop town of Dharamshala his home after he fled his homeland following an abortive anti-Chinese uprising in the Himalayan region in 1959.

Analysts said they were surprised by the Dalai Lama's comments on Burma, where Buddhist monks led a pro-democracy uprising last summer.

"Just like the West, the patience of His Holiness also seems to be running out with the military junta," said Delhi University political analyst Anand Ojha.