Flash floods and landslides have killed at least nine people and forced thousands into temporary shelters on Indonesia's Sumatra island, the local disaster agency said Thursday.

Torrential rain in North Sumatra this week sparked the disaster, with most victims drowning or hit by logs swept away in the current, the agency added.

"We suspect (two victims) were killed after getting hit by logs," said Safaruddin Ananda Nasution, head of Central Tapanuli's disaster mitigation agency.

Rampant illegal logging in the area may have contributed to the disaster by loosening the soil and making it susceptible to landslides, he added.

Several thousand residents have fled to shelters.

This month, record rains triggered flooding and landslides that killed nearly 70 people in and around Jakarta, which is on neighbouring Java island.

Entire neighbourhoods in Indonesia's capital - a megalopolis home to around 30 million people - were submerged in floodwaters that forced tens of thousands into shelters.