The Maritime and Port Authority of Singapore (MPA) is searching for five crew members from a barge that capsized off Horsburgh Lighthouse on Pedra Branca.
On Thursday morning, the MPA received a report that a life raft with people on board had been sighted south-west of Horsburgh Lighthouse.
The Police Coast Guard subsequently rescued six crew members from the barge Heng Hong 168, which had capsized in Singapore waters about 5.5 km or three nautical miles west of Pedra Branca.
The MPA is coordinating the search and rescue operation for the remaining five crew members, it said in a statement on Thursday afternoon. The operation involves air and surface searches, as well as commercial divers. Other vessels in the vicinity have been told to keep clear of the search-and-rescue area and to report any sightings of the missing persons. The MPA is investigating the incident.