YANGON (AFP) - - Aid workers battled Wednesday to get food and water to desperate cyclone survivors in Myanmar, whose government is under fire after more than 22,000 people died in one of Asia's worst natural disasters.

More than 41,000 people are also missing, but the United Nations said foreign staff were still awaiting visas from the secretive military regime -- which said outside aid workers needed to "negotiate" to enter the country.

Hundreds of Buddhist monks were on the streets of Yangon on Wednesday helping residents clear roads of fallen trees and other debris.

The gathering of cinnamon-robed monks was one of the largest groups seen in Myanmar's main city since September when they led mass anti-government protests that were violently put down by security forces.

"We are now relying on monks to clear this road," said one middle-aged woman who lived in the neighbourhood of western Yangon.

"Of course we were hoping the authorities would come, but they haven't shown up yet. These monks came after the storm to help the people to clear the streets and to remove the trees," she told AFP, speaking on condition of anonymity for fear of reprisal from the military government.

Tropical cyclone Nargis barrelled early Saturday into the southwest coast of Myanmar, once known as Burma and now one of the poorest nations on the planet.

UN officials said they were still unable to assess the full extent of the devastation wrought by the storm, especially in the worst-hit Irrawaddy delta region, where entire towns were washed away.

At United Nations headquarters in New York, Rashid Khalikov of the UN Office for the Coordination of Humanitarian Affairs pleaded with the junta to open its borders to foreign aid.

"We cannot tell you are how many people are in need of assistance...It probably will be in the hundreds of thousands," he told reporters.

"We are trying to get maximum cooperation from the government in terms of visas and customs regulations. We really hope it will happen very quickly. We applied for visas. We have not got the visas."

"Let the United States come to help you," said US President George W. Bush, leading international calls to let in foreign relief teams.

Save the Children, one of the few relief agencies allowed to operate in the secretive and impoverished Southeast Asian country, said the toll would rise sharply in the coming days as more victims were found in hard-to-reach areas.

"I wouldn't be surprised if it went as high as 50,000," spokesman Dan Collinson told AFP.

State television said 21,793 people were killed and 40,695 were missing in Irrawaddy division, while 671 were killed and 359 people were missing in Yangon, Myanmar's biggest city and the former capital.

In the delta town of Bogalay, 95 percent of the homes were destroyed, according to Social Welfare Minister Maung Maung Swe.

"Many people were killed in a 12-foot (3.5-metre) tidal wave," the minister told reporters.

Satellite images from US space agency NASA showed virtually the entire coastal plain of the country under water.

Video footage of the disaster zone showed flattened villages, smashed bridges, and survivors forced to live out in the open.

The UN's World Food Programme said it had begun distributing 800 tonnes of food to the hardest-hit areas including Yangon, but that many coastal regions remained cut off due to flooding and road damage.

The United States -- usually one of the junta's toughest critics -- upped its total emergency aid offer to 3.25 million dollars.

"The United States has made an initial aid contribution, but we want to do a lot more," he added as he signed a law giving Myanmar democracy icon Aung San Suu Kyi the Congressional Gold Medal, the US Congress's top civilian honour.

A US disaster relief team was on standby, and the Pentagon said four naval ships currently participating in military exercises off the coast of Thailand could be redirected to Myanmar, if the junta gave the go-ahead.

"But that's all we can do at this point, is to plan, because we have not received a request from the Burmese government," Pentagon press secretary Geoff Morrell said.

Richard Horsey, an OCHA spokesman in Bangkok, warned of the spread of disease, should survivors fail to receive clean drinking water.

"Getting it out to the affected populations will be a major challenge, given that there is widespread flooding," he said.

"The urgent need is for shelter and for water. Without clean drinking water, the risk of disease spreading is the most serious concern."

Despite the widespread devastation, the junta defied calls to postpone Saturday's referendum on a new constitution -- part of its slow-moving "road map" to democracy -- saying it would proceed except in the worst-hit areas.