The UN chief on Saturday warned Asia to quit its "addiction" to coal, as climate change threatens hundreds of millions of people vulnerable to rising sea levels across the region.

The warning follows fresh research this week predicting that several Asian megacities, including Bangkok, Ho Chi Minh City and Mumbai, are at risk of extreme flooding linked to global warming.

Antonio Guterres said Asian countries need to cut reliance on coal to tackle the climate crisis, which he called the "defining issue of our time".

"There is an addiction to coal that we need to overcome because it remains a major threat in relation to climate change," he told reporters ahead of a meeting of the Association of Southeast Asian Nations (ASEAN) in Bangkok on Saturday.

He said countries in the region need to be on "the front line" of the fight by introducing carbon pricing and reforming energy policies.

"We are lagging behind," he said, adding that the rollback of coal could help curb rising global temperatures.

Coal remains a major source of power across Southeast Asia, where breakneck economic development has spurred soaring energy demands - but at a cost to the environment.

About one-third of Vietnam's energy comes from coal power with a slew of new plants set to come online by 2050, while Thailand is investing in fossil fuels. Coastal areas across Southeast Asia have already seen major floods and seawater incursion linked to climate change.

New research this week showed that at least 300 million people worldwide are living in places at risk of inundation by 2050, a much bleaker picture than previous data predicted.

Destructive storm surges fuelled by increasingly powerful cyclones and rising seas will hit Asia hardest, according to the study in the journal Nature Communications.

The UN chief also spoke on Myanmar's persecuted Rohingya Muslims, nearly three-quarters of a million of whom were driven into Bangladesh in 2017.

He urged Myanmar's government to "address the root causes of displacement and allowing of the return, voluntary and in safety and dignity" to Myanmar. "Some steps have been done but they are too small. We need to do much more," he said.

Myanmar's de facto leader Aung San Suu Kyi is also in Bangkok for the summit and is likely to face pressure over her country's treatment of the Rohingya, particularly from Muslim-majority Malaysia and Indonesia.

Myanmar has rebuffed all international pressure so far while only hundreds of Rohingya have returned to Myanmar, due to fear of further repression.