SINGAPORE, July 21 (Reuters) - Recovery from a cyclone that tore into Myanmar's Irrawaddy delta in May, leaving at least 138,000 dead or missing, will cost more than $1 billion, a report by the United Nations and Southeast Asian nations concluded.

The estimate, released on Monday at a meeting of foreign ministers of the Association of South East Asian Nations, covers the most urgent needs such as food, agriculture and housing for the next three years.

"While significant progress has been made to date, we are still in the relief phase for this aid operation," John Holmes, U.N. under-secretary general for humanitarian affairs, told reporters.

Holmes is due to visit Myanmar for three days this week to check on aid delivery to the isolated country that initially shut out foreign relief workers after the deadly cyclone.

The United Nations appealed earlier this month for more than $300 million in additional aid for the former Burma, on top of $178 million already provided by donors.

Myanmar's secretive military government lifted restrictions on foreign aid workers after a visit in late May by U.N. Secretary-General Ban Ki-moon. (Reporting by Jan Dahinten and Melanie Lee, editing by Neil Chatterjee and Roger Crabb)

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