Separate weather systems brought torrential, extended rainfall to the entire island of Sri Lanka and large parts of Indonesia's Sumatra, southern Thailand and northern Malaysia last week.

Much of the region is currently in its monsoon season but climate change is producing more extreme rain events and turbocharging storms.

The World Health Organization said it was deploying rapid response teams and critical supplies to the region.

The UN agency's chief Tedros Adhanom Ghebreyesus told reporters in Geneva that it was "another reminder of how climate change is driving more frequent and more extreme weather events, with disastrous effects".

The relentless rains left residents clinging to rooftops awaiting rescue by boat or helicopter, and cut entire villages off from assistance.

Arriving in North Sumatra on Monday, Indonesian President Prabowo Subianto said "the worst has passed, hopefully".

The government's "priority now is how to immediately send the necessary aid", with particular focus on several cut-off areas, he added.

Prabowo is under increasing pressure to declare a national emergency in response to flooding and landslides that have killed at least 593 people, with nearly 470 still missing.

Unlike his Sri Lankan counterpart, Prabowo has also avoided publicly calling for international assistance.

The toll is the deadliest in a natural disaster in Indonesia since a massive 2018 earthquake and subsequent tsunami killed more than 2,000 people in Sulawesi.

The government has sent three warships carrying aid and two hospital ships to some of the worst-hit areas, where many roads remain impassable.

In North Aceh, 28-year-old Misbahul Munir described walking through water that reached his neck to get back to his parents.

"Everything in the house was destroyed because it was submerged," he told AFP.

"I have only the clothes I am wearing," he said in tears.

"In other places, there were a lot of people who died. We are grateful that we are healthy."

- 'Everything went under' -

In Sri Lanka, the government called for international aid and used military helicopters to reach people stranded by flooding and landslides triggered by Cyclone Ditwah.

At least 366 people have been killed, Sri Lankan officials said on Monday, with another 367 still missing.

Floodwaters in the capital Colombo peaked overnight.

Now that the rain has stopped, there were hopes that waters would begin receding. Some shops and offices have reopened.

The floodwaters came as a surprise to some around Colombo.

"Every year we experience minor floods, but this is something else," delivery driver Dinusha Sanjaya, 37, told AFP.

"It is not just the amount of water, but how quickly everything went under."

Officials said the extent of the damage in the worst-affected central region was only just being revealed as relief workers cleared roads blocked by fallen trees and mudslides.

President Anura Kumara Dissanayake has declared a state of emergency to deal with what he called the "most challenging natural disaster in our history".

The Sri Lankan president received a phone call from India's Prime Minister Narendra Modi on Monday to assure him of New Delhi's continued support for relief and recovery efforts, Indian officials said.

The losses and damage are the worst in Sri Lanka since the devastating 2004 tsunami that killed around 31,000 people there and left more than a million homeless.

- Anger in Thailand -

By Sunday afternoon, rain had subsided across Sri Lanka but low-lying areas of the capital were flooded and authorities were bracing for a major relief operation.

Military helicopters have been deployed to airlift stranded residents and to deliver food. One crashed just north of Colombo on Sunday, killing the pilot.

The annual monsoon season often brings heavy rain, triggering landslides and flash floods.

But the flooding that hit Indonesia, Thailand and Malaysia was also exacerbated by a rare tropical storm that dumped heavy rain on Sumatra island in particular.

The waves of rain caused flooding that killed at least 176 people in southern Thailand, authorities said Monday, one of the deadliest flood incidents in the country in a decade.

The government has rolled out relief measures, but there has been growing public criticism of the flood response, and two local officials have been suspended over their alleged failures.

Across the border in Malaysia, where heavy rains also inundated large stretches of land in Perlis state, two people were killed.

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