Myanmar lowered its flags on Tuesday to begin three days of mourning for about 133,000 people, dead or missing in the cyclone, as Southeast Asia set to work coordinating a much-needed relief effort.

In one of the first demonstrations of public grief since the tropical storm pummelled swathes of this impoverished nation, 18 days ago, national flags in front of Yangon's City Hall fluttered at half mast in the light morning rain.

There was no minute of silence for the victims or a public ceremony, as pressure mounted on the regime to scale up the relief effort for two million survivors in desperate need of food, shelter and medicine.

Myanmar agreed at regional talks in Singapore on Monday to allow the Association of Southeast Asian Nations to coordinate an international relief effort, after resisting repeated foreign bids to deliver aid to hard-hit areas.

Despite the compromise with ASEAN, the regime has yet to soften its refusal to allow in foreign aid workers in the numbers needed to reach the survivors, even in the face of warnings that people could die without help.

"The humanitarian community indicates that 500,000 people currently receive some form of international assistance," the UN said in its latest internal report.

"This is substantially less than the 2.4 million estimated to be affected."