At least 22 people have died and many are missing after heavy rain unleashed flash floods and landslides in western Indonesia, an official said Saturday.

Heavy downpours the week before triggered flooding and landslides that hit several districts on Sumatra island.

At least 17 people were confirmed killed in the last three days in North Sumatra province, while five others were killed in West Sumatra, authorities said.

“Eleven students in Mandailing Natal district, North Sumatra died Friday afternoon during class as they were struck by a building which was destroyed by the flash floods,” spokesman for the national disaster agency Sutopo Purwo Nugroho said.

Dozens of houses have also been destroyed.

“The disaster and search and rescue agency have gone to the districts but have been hampered by the landslides in several areas,” said the head of North Sumatra disaster agency Riadil Lubis.

Landslides and flooding are common in Indonesia, a country prone to natural disasters and torrential downpours.

In June 2016, nearly 50 people died when heavy downpours sent torrents of water, mud and rock into villages also in Central Java province.