YANGON, Myanmar - International aid began to trickle into Myanmar, but the stricken Irrawaddy delta, the nation's rice bowl where 22,000 people perished and twice as many are missing, remained cut off from the world.

With as many as 1 million left homeless after Cyclone Nargis hit over the weekend, the international community was struggling to deliver aid in the military-ruled country, which normally seeks to shut out foreign officials and restricts their access inside the country.

The U.N.'s World Food Program said late Tuesday it has begun distributing aid in damaged areas of Yangon, the largest city, where 800 tons of food had arrived.

The WFP said some villages have been almost totally eradicated and vast rice-growing areas are wiped out by Cyclone Nargis, which hit Myanmar early Saturday.

Images from state television showed large trees and electricity poles sprawled across roads and roofless houses ringed by large sheets of water in the Irrawaddy River delta region, which is regarded as Myanmar's rice bowl.

Buddhist monks and Catholic nuns wielding knives and axes joined Yangon residents Tuesday in clearing roads of ancient, fallen trees that were once the city's pride. And soldiers were out on the streets in large numbers for the first time since the cyclone hit, helping to clear trees as massive as 15 feet in diameter.

Britain said it will contribute up to 5 million pounds (US$9.8 million; �6.4 million) in initial relief funds and also will send an emergency field team to help with international relief efforts and support foreign aid staff already in Myanmar.

U.S. President George W. Bush called on Myanmar's military junta to allow the U.S. to help. The White House said the U.S. will send more than $3 million to help cyclone victims, up from an initial emergency contribution of $250,000.

Bush said Washington was prepared to move naval assets to help search for the dead and missing.

The U.S. Navy has three ships as well as troops in the Gulf of Thailand, within an easy sail of Myanmar, as part of joint military exercises code-named Cobra Gold scheduled for May 8-21. Thailand, Japan, Indonesia and Singapore will also take part in the annual war games.

The Myanmar military, which regularly accuses the United States of trying to subvert the regime, is unlikely to allow U.S. military presence in its territory.

The junta has signaled it will welcome aid supplies for victims of a devastating cyclone, the U.N. said Tuesday, clearing the way for a major relief operation from international organizations.

But U.N. workers were still awaiting their visas to enter the country, said Elisabeth Byrs of the U.N. Office for the Coordination of Humanitarian Affairs.

"The government has shown a certain openness so far," Byrs said. "We hope that we will get the visas as soon as possible, in the coming hours. I think the authorities have understood the seriousness of the situation and that they will act accordingly."

Some aid agencies reported their assessment teams had reached some areas of the largely isolated region but said getting in supplies and large numbers of aid workers would be difficult.

The cyclone came only a week ahead of a key referendum on a constitution that Myanmar's military leaders hoped would go smoothly in its favor, despite opposition from the country's feisty pro-democracy movement. However, the disaster could stir the already tense political situation.

State radio also said that Saturday's vote would be delayed until May 24 in 40 of 45 townships in the Yangon area and seven in the Irrawaddy delta, which took the brunt of the weekend storm. It indicated that the balloting would proceed in other areas as scheduled.

The decision drew swift criticism from dissidents and human rights groups who question the credibility of the vote and urged the junta to focus on disaster victims.

Myanmar's generals have hailed the referendum as an important step forward in their "roadmap to democracy."

But critics, including the United Nations, the United States and human rights groups, question whether it will lead to democracy.

Myanmar has been under military rule since 1962. Its government has been widely criticized for suppression of pro-democracy parties such as the one led by Suu Kyi, a Nobel Peace Prize laureate who has been under house arrest for almost 12 of the past 18 years.

At least 31 people were killed and thousands more were detained when the military cracked down on peaceful protests in September led by Buddhist monks and democracy advocates