The Liberian-registered Niko Ivanka left the capital Monrovia on Saturday for a port in the West African country's south, despite being under a detention order for failing to meet basic safety requirements.
The vessel sent out a distress signal that afternoon, notifying the coastguard that it had taken on water. By the time authorities arrived, it had already partially sunk.
Eighteen people were on the ship's manifest, 11 of whom had been rescued during a 36-hour search, commissioner Eugene Nagbe told reporters on Monday.
But those rescued said that there were in fact about 28 people on board, leaving 17 missing.
"For now, our concern is a search and rescue mission," Nagbe said. "My focus is the lives of those people missing."
The precise number of passengers remained unknown, Deputy Information Minister Jarlaywah Tonpoe told Reuters earlier.
"The vessel was not a passenger-authorized vessel and yet it had passengers on board," Tonpoe said. "So in the coming days, investigation will establish how many people were on board."
It was not clear how or why the ship was able to leave harbor or whether it was carrying any cargo at the time. The vessel's owner, a Chinese national, was arrested on Sunday afternoon, Nagbe said.
Among those listed on the manifest were a Swedish captain, a Chinese crew member, and nine members of West Africa's regional school examinations body.