Flooding and mudslides triggered by monsoon rains have left at least 91 people dead and more than 100 missing in Sri Lanka, officials say, according to BBC News.

About 20,000 have been forced from their homes in the south and west of the country, rescue agencies said.

Military boats and helicopters have been sent to help rescue operations.

The flooding is believed to be the worst since May 2003 when a similarly powerful south-west monsoon destroyed 10,000 homes and killed 250 people.

Sri Lanka's Disaster Management Centre (DMC) said the death toll was rising as reports filtered in from outlying areas.

"The south-west monsoon began with very intense rain," Dunesh Gankanda, deputy minister for disaster management, told reporters in the capital, Colombo.

"There are some areas where we are unable to reach, but relief operations are under way."

One of the worst affected areas is reported to be the south-western district of Kalutara. Police spokesman Priyantha Jayakody said at least five mudslides had been reported there.

The DMC issued evacuation notices to residents along the Kelani river, which flows through Colombo, amid fears that it was about to burst its banks.

"Residents who are living along the Kelani should evacuate to high ground within six hours," the agency said.

As the death toll rose, the government appealed to the UN and to neighbouring countries for assistance.

"The (foreign) ministry will continue to monitor the flood situation and seek assistance as required in consultation with the ministry of disaster management," the government said in a statement.