The death toll from flash flooding and landslides on the Indonesian island of Sumatra has risen to 21, an official said Sunday, with six people still missing.

Torrential rains on Thursday triggered the disaster in Pesisir Selatan regency in the West Sumatra province, with more than 75,000 people forced to evacuate.

"As of Sunday, 21 people were found dead and six people remained missing," Fajar Sukma, an official from West Sumatra disaster mitigation agency, told AFP by phone on Sunday.

A village located on a hillside in the Sutera subdistrict was struck hard, with around 200 families in the area left isolated after a landslide followed by flash flooding, Fajar said.

A local official earlier put the death toll at 18 with five missing.

Rescuers were searching for the missing on Sunday as authorities focused their operation on three areas affected by the disasters, local search and rescue official Abdul Malik said.

"Today’s search involves around 150 people from disaster organisations in West Sumatra," Abdul said in a statement.

Doni Gusrizal, a senior official from the Pesisir Selatan disaster mitigation agency, said waters had started to recede after the flooding, but added that access to areas affected by the landslide remained difficult because of hilly terrain.

In the Padang Pariaman regency, also in West Sumatra, heavy downpours earlier this week caused rivers to overflow and triggered floods and a landslide, killing at least three people, according to a statement from the local disaster agency.

Indonesia is prone to landslides during the rainy season and the problem has been aggravated in some places by deforestation, with prolonged torrential rain causing flooding in some areas of the archipelago nation.

A landslide and floods swept away dozens of houses and destroyed a hotel near Lake Toba on Sumatra in December, killing at least two people.