JAKARTA - A ferry sank in a river in East Kalimatan of Indonesia before midnight Thursday, leaving 26 people missing, search and rescue operation has already been underway, National Search and Rescue Office said Friday.

KM Surya Indah ship carrying 97 people capsized in Mahakam river located between Melak and Maura Paku at 23:00 pm local time Thursday, said Gusti Ayu Ketut Jayanti, a press officer at the office. But the number of people on board in Indonesia often exceeds the number in the manifest.

"As many as 71 people have been evacuated, but the 26 others remain missing," she told Xinhua by phone.

Search and rescue operation had been underway, involving those from local search and rescue office, the military and the police, said Ayu.

It was still unclear what caused the accident, she said. ?

Water transport is one of favorite transport means in the vast archipelago country, but lacking of safety has frequently triggered accidents.