Hours of heavy rain caused flooding in two districts in West Sumatra province on Saturday evening and sent sweeping ash and large rocks down Mount Marapi, the most active volcano in Sumatra.

"Until now our data shows that 34 people died: 16 in Agam and 18 in Tanah Datar. At least 18 others are injured. We are also still searching for 16 other people," West Sumatra disaster agency spokesman Ilham Wahab told AFP.

He said the search effort involved local rescuers, police, soldiers and volunteers.

Agam and Tanah Datar districts were hit with flash floods and cold lava flow at around 10:30 pm (1530 GMT) on Saturday, according to Basarnas search and rescue agency.

Cold lava, also known as lahar, is volcanic material like ash, sand and pebbles carried down a volcano's slopes by rain.

Earlier, Basarnas said 12 people had died including several children.

Ilham said Sunday that authorities were still receiving reports of missing people from relatives.

He said he could not give a figure of the number of people evacuated from the area as the search and rescue effort was still focused on the victims and the missing.

Mosques, houses damaged 

In the Tanah Datar district, several mosques and a public pool were damaged in the area with large rocks and logs scattered on the ground, according to an AFP journalist at the scene.

BNPB said in a statement that flooding since Saturday night had brought mud to the Tanah Datar regency, affecting five sub-districts.

BNPB said its latest assessment showed 84 housing units and 16 bridges were affected by the disaster, adding that heavy equipment had been deployed to speed up the clearing of road access.

In Lembah Anai, a popular tourist spot with a waterfall in Tanah Datar, a road connecting the cities of Padang and Bukittinggi was severely damaged and access for cars was cut off. 

At a nearby river, two trucks had been swept away by the flooding and strong river currents, the journalist said.

In Agam, dozens of homes and public facilities were damaged, the district's disaster agency chief Budi Perwira Negara told AFP.

Nine bodies including those of a three-year-old and eight-year-old were identified earlier on Sunday, head of the local rescue agency Abdul Malik said in a statement.

Authorities dispatched a team of rescuers and rubber boats to look for the missing victims and to transport people to shelters.

The local government set up evacuation centres and emergency posts in several areas of Agam and Tanah Datar districts.

Indonesia is prone to landslides and floods during the rainy season.

In March at least 26 people had been found dead after landslides and floods hit West Sumatra.