At least 43 people died in flash floods triggered by heavy rains and torrents of cold lava and mud flowing down a volcano's slopes on Indonesia's Sumatra island.

Officials on Monday said another 17 people remained missing as the search and rescue operation continued.

Torrential rain on Saturday caused flash floods, landslides, and cold lava flow from Mount Marapi in three districts in West Sumatra province.

The deluge tore through mountainside villages and swept away people while submerging nearly 200 houses and buildings and damaging rice fields, said Abdul Malik, the chief of the provincial rescue team.

Indonesian rescuers raced on May 15 to find dozens of people still unaccounted for after heavy rains caused flash floods and washed volcanic debris into residential areas over the weekend, sweeping away houses and leaving 67 people dead.

Hours of torrential rain on May 11 caused mud and rocks to flow into districts near one of Indonesia's most active volcanos, destroying dozens of houses and damaging roads and mosques.

"Some of the missing ones have been found. According to the police identification, 67 people died," national disaster agency chief Suharyanto said on May 15 in a press conference, raising the death toll from 58.

He added that 20 people remained missing, with rescuers saying many of the retrieved bodies were found in or around rivers after being swept away by the deluge of volcanic material, mud and rain.

© ReutersMen standing near a damaged house in Tanah Datar in the West Sumatra province.

The mixture of ash, sand and pebbles carried down a volcano's slopes by rain is known in Indonesia as lahar, or cold lava.

Heavy equipment was deployed to clear debris from the areas worst hit by flooding and cold lava flows, which have affected transport access in six districts, said Mr Suharyanto, who goes by one name.

"We have been scouring the areas that cannot be reached with cars. The currents in the river are extreme, so the search is quite dangerous," Mr Ritno Kurniawan, a 38-year-old member of the local rafting community, told AFP.

"We usually find the bodies along the riverbanks, buried under volcanic material or rocks."

More than 3,300 people have been forced to evacuate from affected areas.