Helicopters, navy boats and thousands of troops are being used to evacuate stranded villagers in several parts of the country, the Disaster Management Centre (DMC) said.

Rain was falling across the island's entire 65,000-square-kilometre (25,000-square-mile) area, with some regions receiving 360 millimetres in the past 24 hours, the DMC said.

The Kelani River, which flows into the Indian Ocean near the capital Colombo, was expected to breach its banks later on Friday and residents were warned to move to higher ground.

Resident V. S. A. Ratnayake, 56, said he had to leave his flooded home in Kaduwela, just outside Colombo.

"I think this could be the worst flood in our area for three decades," Ratnayake told AFP. "I remember a flood in the 1990s when my house was under seven feet of water."

The army has deployed more than 20,000 troops, while the navy was using boats to evacuate villagers and to help clear debris.

Of the 56 people confirmed dead, 26 were buried alive in mudslides in the tea-growing Badulla district in the central region, the DMC said. Another 21 are missing, with 14 more in hospital.

Nearly 3,000 homes were damaged in mudslides and floods, and almost 15,000 people had been moved to temporary shelters.

- Climbed a coconut tree -

In Anuradhapura district in the north, an Air Force Bell 212 helicopter airlifted a man who had climbed a coconut tree to escape rising waters.

Video footage released by the military showed similar rescues taking place across the country.

The DMC said more rain was forecast, with Cyclone Ditwah likely to move away from the north towards the south Indian state of Tamil Nadu by Sunday.

Officials from the agency said they expected flood levels to be worse than in 2016, when 71 people were killed nationwide.

Another resident, M. A. Madushantha, 38, said floodwaters rose before his eyes and inundated his home on the banks of the Kelani.

"I sent my children and wife to her sister's house last night because we expected this," Madushantha told AFP.

- 'Nowhere to go' -

The Sirasa TV network broadcast an appeal for help from a stranded woman in the central region of Ruwanwella.

The woman, identified only as Akma, said she was upstairs in a two-storey home, with the ground floor completely flooded.

"We are six people, including a one-and-a-half-year-old child. If the water rises another five steps up the staircase, we will have nowhere to go," she said by telephone.

Transport Minister Bimal Rathnayake said 60 Indian tourists who were stranded in central Sri Lanka were taken to Colombo on Friday.

Sri Lanka is in its northeast monsoon season, but rainfall has intensified because of Cyclone Ditwah, the DMC said.

Sri Lanka depends on seasonal monsoon rains for irrigation and hydroelectricity, but experts have warned that the country faces more frequent floods due to climate change.

The government has suspended final-year school examinations nationwide for two days because of the weather, while civil servants were given a day off.

Parliament also suspended its budget debate so that legislators could return to their constituencies to deal with the damage.

This week's weather-related toll is the highest since June last year, when 26 people were killed following heavy rains. In December, 17 people died in flooding and landslides.

The worst flooding this century occurred in June 2003, when 254 people were killed.