A freighter carrying 560 metric tonnes of bunker fuel sank after colliding with a Cayman Islands-registered chemical tanker in the Singapore Strait.
The accident between the tanker Stolt Commitment and freighter Thorco Cloud, which bears the Antigua and Barbuda flag, happened at 8.14pm in Indonesian waters 11km north-west of Batam.
Six crew members have been rescued and a search is ongoing for others.
There were no Singaporean crew members on either vessel.
The chemical tanker sustained minor damage and is in stable condition, according to a press release from the Maritime and Port Authority (MPA) of Singapore.
MPA says it deployed two patrol craft for search and rescue operations.
Singapore’s Police Coast Guard supported the MPA by deploying five boats, and rescued five of the 12 freighter crew members.
A sixth was rescued by the chemical tanker.
The Singapore Civil Defence Force (SCDF) dispatched two vessels to assist in the search and rescue efforts.
SCDF also took the six rescued crew members to Singapore General Hospital.
Search and rescue operations for the remaining freighter crew members are on-going.
MPA has alerted the Indonesian authorities to the incident.
The Indonesian authorities have also commenced search and rescue operations.
MPA is assisting the Indonesian authorities to survey and mark the sunken freighter to ensure navigational safety, and has issued navigational broadcasts to vessels in the vicinity to keep clear of the area and to report any sightings of the missing crew members.
There is no disruption to shipping traffic in the Singapore Strait, it said..
While there are currently no reports of any oil spill, MPA has also put on standby anti-pollution craft.