Floods and landslides in Sri Lanka's southern and western regions have killed at least 122 people and damaged more than 1,700 homes, officials said on Saturday.

Ninety-seven people were also reported missing, at least 40 others have been hospitalised, and over 400,000 people have been displaced after unusually heavy rain on Friday triggered a string of mudslides and caused rivers to burst their banks, according to the country's Disaster Management Centre (DMC).

Indian medical teams and emergency relief arrived in the capital, Colombo, on Saturday to help Sri Lanka deal with the worst flooding in 14 years.

"Many thousands are displaced and are trying to come to terms with what has happened with this huge deluge of water," Al Jazeera's Minelle Fernandez, reporting from the southern town of Agalawatte, said.

"Some places received a year's supply of rain in 24 hours. It has taken everyone by surprise."

In Agalawatte, some 100km south of Colombo, rescuers pulled at least 11 bodies out from the mud and earth, while one person was found alive.

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"It's been 15-18 years since we've had so much water. It's left people helpless", a man told local media.

"We were moving things from 2am. Kitchen stuff floated off, and the roof shades were blown away," a woman said.

Authorities issued fresh evacuation orders for those living downstream of two major rivers, citing a risk of flooding even though the rains had subsided.

Soldiers have fanned out in boats and helicopters to help with rescue operations. Residents said there are more people trapped in interior villages where boats have been unable to reach.

An Indian ship carrying medical supplies docked in Colombo on Saturday, after Sri Lanka issued an international appeal for help. Another ship is due to arrive on Monday.

The flooding is the worst since May 2003 when 250 people were killed and 10,000 homes destroyed after a similarly powerful Southwest monsoon, officials said.

The DMC said the monsoon ended a prolonged drought that had threatened agriculture as well as hydropower generation.

Mudslides have become common during the monsoon season in the tropical Indian Ocean island as land has been heavily deforested to grow export crops such as tea and rubber.

Last May, a massive landslide killed more than 100 people in central Sri Lanka.