UN Secretary General Ban Ki-moon, centre, tours the Shwedagon pagoda before meeting with Burmese Prime Minister Thein Sein and viewing the cyclone-hit Irrawaddy Delta on Thursday. (Stan Honda/Associated Press)

The head of the United Nations said he was "very upset" Thursday after flying over Burma's flooded Irrawaddy Delta on Thursday to view the damage left by a devastating cyclone that has left millions in desperate need of help.

Secretary General Ban Ki-moon's visit to Burma, also known as Myanmar, is part of the UN's effort to convince the country's reclusive military rulers to increase the amount of foreign aid entering the country following Cyclone Nargis.

"I'm very much concerned," a visibly shaken Ban said following the tour. "Many human lives have been lost, houses are destroyed, roads and streets are washed away, and all rice paddies flooded with water."

He earlier told reporters upon his arrival Thursday that this period of time following the cyclone represents a "critical moment for Myanmar."

"The United Nations and all the international community stand ready to help to overcome the tragedy. The main purpose of my being here is to demonstrate my solidarity," Ban said.

The UN estimates that nearly 2.5 million people have been stranded without food, water or shelter after the May 3 storm, which left nearly 134,000 dead or missing.

Ban said that while the UN has a functioning relief operation in place, so far it has been able to reach only 25 per cent of those affected by the storm.

On his first day in the southeast Asian nation, Ban met with Burmese Prime Minister Lt.-Gen. Thein Sein, telling him foreign aid experts must be allowed into the country as soon as possible because the crisis had exceeded Burma's capacity, according to a UN official at the talks.

During the 1.5-hour meeting, the UN official said Sein told Ban that the relief phase of the government's operation was ending and efforts were being centred on reconstruction — despite reports from the International Red Cross that corpses were still floating in waterways around the Bogale delta area, where many are yet to receive aid.

Government and aid officials and private-sector donors from 29 countries, including Japan, Singapore and Thailand, will visit the region Friday.

"The UN Secretary General is walking a very, very fine tightrope," the CBC's Stephen Puddicombe reported from Bangkok on Thursday.

"He has to push to get more aid into Burma, while not upsetting Burma's military leaders. [The] government wants $11 billion US but doesn't want to let donors in. The international donors won't give over the money unless they can verify the needs, which means they have to get into the country."

Burma's military junta has been criticized for its reluctance to allow foreign aid and global relief teams into the country. On Wednesday, the Burmese government said it would not accept aid for cyclone victims from U.S. warships.

World Food Program officials in Bangkok said they would begin Thursday sending some of the 10 helicopters approved by Burma to deliver aid to the hardest-hit areas in the Irrawaddy Delta area, instead of dropping it off at the airport in Rangoon.

It was not immediately clear when the helicopters, each capable of carrying nearly three tonnes of supplies, would arrive.

"We are doing everything we can to get them in as soon as possible," said WFP official Marcus Prior.

UN official Dan Baker said Ban will meet junta leader Senior Gen. Than Shwe on Friday in Naypyidaw, the capital built by the military in a remote area of central Burma. Before Ban's arrival, his letters and telephone calls to Shwe had gone unanswered for days. The junta announced its approval of the secretary general's visit on Monday.

Makes visit to spiritual heartland

Ban has already visited Shwedagon pagoda, considered Burma's spiritual heartland, where he was accompanied by Foreign Minister Nyan Win.

"I hope your people and government will closely co-ordinate so that the flow of aid and aid workers' activities can be carried out in a more systematic way," he said.

Britain, France and the U.S. all have naval vessels stocked with relief supplies near the coast of Burma, but are waiting for approval from the government to deliver and distribute the goods. The state-run newspaper, The New Light of Myanmar, said the U.S. is not welcome in the southeast Asian country, despite the fact that it has already accepted 40 aid flights by U.S. military C-130 cargo planes.

The newspaper said accepting military-linked assistance "comes with strings attached" that are "not acceptable to the people of Myanmar."

Pentagon spokesman Bryan Whitman said the four U.S. ships will stay put until further notice.

"It's very hard to see that type of suffering that's going on ... and to turn your back and leave," Whitman said.

International aid agencies and foreign governments have been urging the reclusive army generals who run Burma to allow more foreign aid and global relief teams into the country. What aid has been allowed in is being delivered solely by Burmese army teams, amid accusations of corruption and favouritism. Very few foreign relief experts have been given visas.

The junta has been gradually relaxing some of its restrictions on foreign aid, announcing it would allow medical teams from neighbouring southeast Asian countries into the cyclone zone and giving a few visas to some United Nations and other agencies.

Student activists in northern Thailand, just across the border with Burma, said the UN and international governments aren't doing enough to make sure aid gets in, Puddicombe reported.

"They said bluntly to me, 'The UN is useless. Countries like the U.S. and the U.K. should get off their butts and force aid into the country, what's Burma going to do about it? Where people are dying, just do it and stop talking.' And there wasn't a lot to ask them after that," he said.

Other activists have called on Ban to meet with detained pro-democracy leader Aung San Suu Kyi and try to secure her release. The Nobel Peace prize laureate has been under house arrest in Rangoon for most of the last 18 years and her current period of detention is due to expire Monday.

With files from the Associated Press