Flash floods and landslides triggered by torrential rains in Indonesia have killed at least 21 people, including 11 schoolchildren, left 15 missing, and destroyed hundreds of homes, authorities said on Saturday.

More than 500 homes in the provinces of North and West Sumatra have been flooded or damaged, with some swept away by the floods, which also destroyed three suspension bridges, said a disaster relief official.

"Evacuation, as well as search and rescue operations, are underway," said Sutopo Purwo Nugroho, a spokesman for disaster mitigation agency BNPB. "But the affected villages are in the mountains and access is difficult, due to damaged roads."

In North Sumatra, 11 children studying at an Islamic village school died after their classroom wall collapsed when a nearby river overflowed on Friday.

"The victims were buried in a torrent of mud and wall debris," Sutopo added.

Rescuers are hunting for one student still missing from the 29 in the class at the time, but have accounted for all the rest, regional police chief Irsan Sinuhaji told Reuters, adding that authorities were checking for other people who may have gone missing.

Two people were found dead on Saturday after their vehicles were swept away by the river.

Four people died in landslides in the city of Sibolga in North Sumatra, while flash floods in West Sumatra killed four more, including two children.