At least 21 people have died and dozens more are missing in devastating floods that have engulfed thousands of homes in Indonesia.

Rivers turned into raging torrents as heavy rain inundated the South Sulawesi province, displacing more than 15,000 residents.

Rescuers are searching for 23 people who are missing.

The number of fatalities is expected to continue to climb as soldiers, police and civilian groups dig through thick mud up to two metres deep.

The heavy rain, which caused three rivers to overflow, is still falling, making the search effort even more difficult.

Cars are stuck in mud after flash flooding in Masamba, South Sulawesi province. Photo: AAP

“We are focusing on the efforts to clean out the debris that inundated the road so that we can open the access to the site to bring logistics,” North Luwu mitigation and rescue agency head Muslim Muchsin said.

“We have received reports from local residents who said their relatives have gone missing – there are dozens of them missing in total.”

Many victims have been found buried under the mud that has submerged thousands of homes in six subdistricts in South Sulawesi.

South Sulawesi deputy governor Andi Sudirman Sulaiman said aid supplies had begun to arrive.

“Our team has entered North Luwu and immediately distributed aid,” he said.

Residents evacuate Radda village in Luwu Utara district. Photo: AAP

More than 2000 residents who sought refuge at temporary shelters have been unable to access emergency supplies because the flash flooding has made many roads inaccessible.

Raditya Jati, from Indonesia’s National Disaster Mitigation Agency, said they were unable to get into the North Luwu district as mud had covered the main road into the area.

“I hope we will be united in our recovery efforts,” Nurdin Abdullah, the governor of South Sulawesi, said on Kompas TV.

Indonesia frequently suffers from floods and landslides, particularly during the rainy season.

-with agencies