Floodwaters running as high as two metres in some areas have hit nine Thai provinces and eight states in neighbouring Malaysia, across a swathe of hundreds of kilometres devastated in 2024 by seasonal monsoon floods that killed 12.
The Thai navy said it was readying to send a flotilla of 14 boats and the aircraft carrier Chakri Naruebet accompanied by helicopters, doctors, supplies and field kitchens that can supply 3000 meals a day.
"The fleet is ready to deliver forces and carry out actions as the Royal Navy orders," it said in a statement on Tuesday, adding that the carrier could also serve as a floating hospital.
An estimated 1.9 million people have been affected in Thailand, where the meteorology agency forecast sustained heavy rain and flash floods on Tuesday and warned small boats to stay ashore to avoid waves taller than 3m.
"Calls have been coming in non-stop in the last three days, in the thousands, asking to be evacuated and others for food," said a member of volunteer group the Matchima Rescue Centre in the worst affected city of Hat Yai.
The rubber trading centre is Thailand's fifth-largest city, where authorities have ordered evacuation after days of rain that Prime Minister Anutin Charnvirakul said had brought the worst flooding in 15 years.
"We are five people and a small child without rice and water," Facebook user The Hong Tep posted in an appeal for help on the Matchima group's page. "Phone reception has been cut - water is rising fast."
Hat Yai, also popular with Malaysian visitors, received 335mm of rain on Friday, its highest in a single day in three centuries.
Television images showed brown waters rushing through its commercial streets, while residents waded through high water, clinging to floating polystyrene boxes as rubber boats evacuated others in orange life vests.
The waters submerged cars and flowed around a fire truck abandoned in a street.
In Malaysia, more than 18,500 people moved from flooded areas to 126 evacuation centres set up mainly in northern border areas.
In the state of Perlis, rescue teams waded through knee-high water to enter homes, while rescue boats ferried the elderly to safety, images from its fire department showed.
A team of rescuers sent to the worst-hit state of Kelantan bordering Thailand could fan out to other states if needed, Prime Minister Anwar Ibrahim said on Facebook.
"Family safety must be the priority," he said, ordering authorities to provide maximum support to affected communities, whom he asked to comply with orders to evacuate.
"In this difficult and challenging time, I pray that all flood victims are granted strength, resilience, and protected from any harm."
Posts from stranded people desperate for help ran into the thousands on the Facebook page of Hat Yai's Matchima rescue group.
"Water is on the second floor now," wrote one of them, Pingojung Ping, who said she was one of six trapped, two elderly people among them. "Pray. Please help."Â
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