Six crew members were still missing last night after their freighter sank in a collision with a chemical tanker in the Singapore Strait on Wednesday night.
The four Filipino and two Ukrainian men had been on board the Thorco Cloud, which bore the Antigua and Barbuda flag.
It collided with Cayman Islands-registered chemical tanker Stolt Commitment in Indonesian waters 11km north-west of Batam at 8.15pm.
The freighter, which was carrying 560 tonnes of bunker fuel, had 12 crew members in total.
Five of them were rescued by the Singapore Police Coast Guard, which had deployed five boats to support the two patrol craft of the Maritime and Port Authority (MPA) in search and rescue operations.
Another crew member was rescued by the chemical tanker, which only sustained minor damage.
The six men who were rescued were taken to Singapore General Hospital, where they were under observation last night.
There were no Singaporeans on either vessel.
The Indonesian authorities have been alerted to the incident, and have commenced their own search and rescue operations.
An MPA spokesman said: "Singapore is ready to render assistance if required."
"Navigational broadcasts were made to all ships to keep a lookout for the missing six crew members. To ensure navigational safety, MPA deployed a buoy tender and a hydrographic survey vessel to cordon the area," he added.
The accident has not disrupted shipping traffic in the Singapore Strait.
Anti-pollution craft have been placed on standby by the MPA, though no oil spills have been reported.
Marine life could be adversely affected in the event of an oil spill, said Assistant Professor Huang Danwei, who teaches marine biology at the National University of Singapore.
"Birds feeding on the surface could be coated with oil and organisms under the water could be suffocated due to a lack of oxygen," he said, adding that a spill could also affect coastal wildlife if allowed to spread.
However, Prof Huang said that the effects of an oil spill could be lessened if cleaned up quickly.
The accident comes almost three weeks after passengers were forced to evacuate a ferry travelling from Singapore to Batam when it struck a floating object, causing water to seep into the vessel.