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.