At least 34 people were killed and 16 more were missing in western Indonesia after flash floods and cold lava flow from one of the archipelago's most active volcanoes damaged homes, roads and mosques, local officials said on Sunday.

Hours of heavy rain caused flooding in two districts -- Agam and Tanah Datar -- in West Sumatra province on Saturday evening, threatening thousands after it swept ash and large rocks down Mount Marapi.

"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.

Abdul Malik, head of Padang Search and Rescue Agency, told reporters three more people had died but they were yet to be confirmed by other authorities.

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

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

About 370,000 people live in Tanah Datar, where several mosques and a public pool were damaged, with large rocks and logs scattered on the ground, according to an AFP journalist at the scene.

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.

Aerial images of Tanah Datar showed roads covered by mud, with roofs and a mosque's minarets jutting out of the sea of brown mud.

Ilham said on Sunday that authorities were still receiving reports of missing people from relatives. (AFP)