* ASEAN to coordinate international relief effort

* U.N.'s Ban to visit Myanmar on Wednesday

* Myanmar declares three days of mourning

* Donor conference in Yangon on May 25 (Adds U.N. secretary general visit on Wednesday)

By Aung Hla Tun

YANGON, May 19 (Reuters) - Southeast Asian nations will take the lead in an international aid effort for cyclone-hit Myanmar, but the military junta will not give Western relief workers unfettered access to disaster areas, Singapore said on Monday.

"We will establish a mechanism so that aid from all over the world can flow into Myanmar," Foreign Minister George Yeo said.

He was speaking after hosting a regional meeting to prod the generals to accept large-scale foreign aid and expertise for up to 2.4 million people left destitute by Cyclone Nargis.

The details were to be worked out with the United Nations, which announced later on Monday that a donor conference would be held in the cyclone-hit former capital, Yangon, on May 25.

Myanmar agreed to accept nearly 300 medical personnel from its neighbours in the 10-member Association of Southeast Asian Nations (ASEAN), the foreign ministers said in a statement.

A few have already sent teams two weeks after the disaster which left 134,000 dead or missing. But aid workers from outside ASEAN will only be granted visas on a case-by-case basis.

"We have to look at specific needs -- there will not be uncontrolled access," Yeo said after the meeting which named ASEAN chief Surin Pitsuwan to work with the United Nations on aid delivery.

U.N. Secretary-General Ban Ki-moon will visit Myanmar on Wednesday, when he plans to visit the country's Irrawaddy delta area which was hit hardest by Nargis, his spokeswoman Michele Montas told reporters.

"His objective is to reinforce the ongoing aid operation, see how the international relief and rehabilitation effort can be scaled up and work with Myanmar authorities to significantly increase the amount of aid flowing through Yangon to the areas most affected by the disaster," Montas said.

"He is also (hoping) to more effectively coordinate and systematize the international community's emergency relief and longer term rehabilitation and reconstruction assistance," Montas said.

Humanitarian agencies say the death toll from Nargis, one of the most devastating cyclones to hit Asia, could soar without a massive increase of emergency food, water, shelter and medicine to the delta, the country's rice bowl.

TRICKLE OF AID

While aid has been trickling into the delta, the U.N. World Food Programme (WFP) says it has managed to get rice and beans to just 250,000 of the 750,000 people it thinks are most in need.

Britain's Asia minister Mark Malloch-Brown said in London on Monday after returning from Myanmar that the authorities and international humanitarian organisations had widely differing views as to immediate needs.

"Getting a needs assessment done in time for the donors' meeting is critical to get everyone on the same page," he told reporters in London. "Unless you have an agreed assessment ... you just get nowhere with the donors' meeting."

In one town in the upper delta, a steady stream of refugees arrived after travelling for days from Pyinsalu, one of the worst-hit districts.

"I didn't have any kids, but I lost all my relatives. It's only my wife and me now," said one man, his clothes soaked by rain and wearing no shoes.

Analysts are making much of reclusive junta supremo Than Shwe's recent appearances in the disaster areas.

On Monday, the former Irrawaddy division commander visited two badly-hit townships in the delta and called for "concerted efforts among the government, the military and the people for the reconstruction of the region," state-run radio said.

On Sunday, state television showed the bespectacled 74-year-old Senior General in Yangon, the city he deserted in 2005 for a remote new capital 250 miles (390 km) to the north. meeting ministers involved in the rescue effort.

"It is not insignificant that he has been forced out of his lair," one Yangon diplomat said. "There are obviously some in the military who see how enormous this is, and how enormously wrong it could go without further support."

On Monday, state radio announced a three-day mourning period for cyclone victims, beginning on Tuesday.

It also reported the U.N.'s chief humanitarian officer, John Holmes, visited devastated Labutta and Bogalay townships with government officials.

Holmes is expected to meet Prime Minister Thein Sein on Tuesday and deliver a message from Ban to the generals.

In the last 50 years, only two Asian cyclones have exceeded the human toll of Nargis -- a 1970 storm that killed 500,000 people in neighbouring Bangladesh and another that killed 143,000 people in 1991, also in Bangladesh.

The United States and France have naval ships equipped with aid supplies and helicopters waiting in international waters off the Myanmar coast, although Paris and Washington say they will not go in with the green light from the generals.

French Foreign Minister Bernard Kouchner said on Monday countries on the U.N. Security Council that did not agree to pressure Myanmar into opening its doors to foreign aid were guilty of "cowardice".

China, Russia, Vietnam and South Africa oppose a Council role in what they say is a humanitarian, not a political issue.

ASEAN, which has a policy of non-interference in each others' affairs, has shunned taking unilateral humanitarian action.

"It doesn't make sense for us to work on the basis of forcing aid on Myanmar because that would bring unnecessary complications and will lead to more suffering for the Myanmar people," Yeo said after the Singapore meeting.

(With additional reporting by Melanie Lee, Neil Chatterjee, Jan Dahinten and Olivia Rondonuwu in SINGAPORE; and Manny Mogato in MANILA; Writing by Ed Cropley and Darren Schuettler; Editing by Jerry Norton)

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