Indonesia, Malaysia, and Thailand have been severely affected, with millions facing disruption.
On Indonesia’s Sumatra Island, the death toll has risen to more than 300, and authorities fear the number may grow as dozens of people remain missing. Rescue teams are still working in several areas where major roads have been cut off and power and internet services are only partly restored.
Thailand has reported 160 deaths as of Saturday, while Malaysia has also confirmed several fatalities linked to the flooding.
The destruction in Indonesia was made worse by a rare tropical cyclone, Cyclone Senyar, which triggered large landslides and washed away homes. Thousands of buildings have been submerged.
Residents described how quickly the disaster struck. Arini Amalia, from Aceh Province, said the floodwaters surged into the streets within seconds. When she returned to her home a day later, she found it completely underwater. “It’s already sunk,” she said.
In West Sumatra, Meri Osman said he was carried away by the powerful current and survived only by clinging to a clothesline until rescuers reached him.
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Evacuation efforts continue across the affected countries as officials warn that more heavy rain may fall in the coming days.
"During the flood, everything was gone," a resident of Bireuen in Sumatra's Aceh province told Reuters news agency. "I wanted to save my clothes, but my house cam down."
The bad weather has hampered rescue operations, and while tens of thousands of people have been evacuated, hundreds are still stranded, the Indonesian disaster agency said.
In Tapanuli, the worst-affected area, reports say residents have ransacked grocery stores in search of food.
Pressure is mounting on Jakarta to declare a national disaster in Sumatra to enable a faster and more coordinated response.
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In Thailand's southern Songkhla province, water rose 3 m (10 ft), and at least 145 people died in one of the worst floods in a decade.
Across the 10 provinces hit by flooding, more than 160 people have been killed, the government said on Saturday. More than 3.8 million people have been affected.
The city of Hat Yai experienced 335mm of rainfall in a single day, the heaviest in 300 years. As waters receded, officials recorded a sharp rise in the death toll.
At one hospital in Hat Yai, employees were forced to move bodies to refrigerated trucks after the mortuary became overwhelmed, news agency AFP reported.
"We were stuck in the water for seven days and no agency came to help," Hat Yai resident Thanita Khiawhom told BBC Thai.
The government has promised relief measures, including compensation of up to two million baht ($62,000) for households that lost family members.
In neighbouring Malaysia, the death toll is far lower, but the damage is just as devastating.
Flooding has wreaked havoc and left parts of northern Perlis state under water, with two people dead and tens of thousands forced into shelters.
Elsewhere in Asia, Sri Lanka has been battered by Cyclone Ditwah, with more than 130 people dead and some 170 missing, officials said.
Sri Lanka is also grappling with one of its worst weather disasters in recent years, and the government has declared a state of emergency.
More than 15,000 homes have been destroyed and some 78,000 people have been forced into temporary shelters, officials said. They added that about a third of the country was without electricity or running water.
Meteorologists have said the extreme weather in Southeast Asia may have been caused by the interaction of Typhoon Koto in the Philippines and the rare formation of Cyclone Senyar in the Malacca Strait.
The region's annual monsoon season, typically between June and September, often brings heavy rain.
Climate change has altered storm patterns, including the intensity and duration of the season, resulting in heavier rainfall, flash flooding and stronger winds.
Source: BBC