Floods and torrents of mud unleashed by heavy rains in Sri Lanka have killed 91 people and left 110 others missing, and authorities have appealed for international help.

Key points:

• More than 61,000 affected by heavy rain that started on Friday
• Mudslides brought on by deforestation for growth of crops
• Aid groups ordered to cancel holidays, be on alert for the next three days

The Disaster Management Centre said 2,040 people were evacuated to safer locations and more than 61,000 were affected by the rain that started early on Friday.

Sri Lanka's Foreign Ministry appealed for assistance from the United Nations and from other countries.

The government advised people living near swollen rivers and hilly slopes prone to landslides to evacuate, as heavy rains were expected to continue.

Navy boats and air force helicopters were deployed to rescue marooned victims and provide emergency relief.

Deputy Minister for Disaster Management, Dunesh Kankanda, said many people were on roofs and treetops calling for help.

Many homes and roads were inundated and schools were closed in the province of Sabaragamuwa, about 90 kilometres east of the capital, Colombo.

Aid groups, rescuers and other government workers were ordered to cancel any holidays and remain on alert for the next three days.

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.