Lalu Wahyu Efendi, search and rescue coordinator in the West Nusa Tenggara provincial capital Mataram, told Kyodo News at least three people were found dead and 46 others survived after the ferry Munawar sank at around 2am on its way from Lombok Island, next to Bali, to nearby Sumbawa Island.
How many people have gone missing, however, was not immediately clear.
"I haven't been able to confirm how many people were aboard when the ferry sank," Lalu said.
Officials and media reports gave conflicting figures on the missing people, ranging from 11 to 40 people. The state-run news agency Antara even quoted passengers as saying the ferry was packed with about 150 people when it sank.
The reason the ferry sank is not yet known, although passengers told local media that leakage had caused the accident.
"The ferry leaned to the left, then the engine didn't work and it sank," Lalu quoted one of the witnesses as saying.
Ferries are a common form of public transportation in Indonesia, particularly in the eastern part of the archipelago, due to bad road conditions in some regions.
Lack of adequate public sea transportation, as well as efforts to increase profits, however, prompt some shipping companies to let passengers and vehicles board already-packed ships and ferries.