At least 21 people have been killed and 31 others reported missing due to the flash floods and landslides in Indonesia's South Sulawesi province, an official said.The search and rescue efforts were intensified in the flash flood-stricken district of Luwu Utara on Wednesday, involving 539 rescuers, the official told Xinhua news agency."Today five bodies were retrieved by the rescuers, bringing the total number of casualties to 21," he said on Wednesday.Meanwhile, 10 people are still under medical treatment in a hospital, he said.Most of the bodies were recovered from the mud which submerged the areas. The mud was 50 centimetre to 1 metre high, he added.The search and rescue mission will resume on Thursday morning to focus on Masamba city and Radda village, as they were hit the hardest by the calamity, the official said."For tomorrow (Thursday), the number of the rescuers have been divided into six teams. They will intensify search and rescue efforts in Masamba and Radda," he said.Heavy downpours triggered river waters in the district to overflow and submerge surrounding areas.Scores of houses, a school building and public facilities were damaged, according to the search and rescue office.The catastrophe has impacted nearly 5,000 people as their houses were hit by the flash floods, according to the National Disaster Management Agency.