Two Indian nationals working below a ship's deck were killed in what Singapore officials believe to be a case of gas poisoning, news reports said on Thursday, the dpa reported.
Five other workers were hospitalized and in stable condition.
All seven Indian nationals working below the deck of the Pacific Ataawhai in a Singapore dry dock on Wednesday found themselves gasping for breath, The Straits Times said.
The two who died, aged 21 and 25, were believed to have suffocated.
It was the second shipyard tragedy in less than a month. A 42- year-old Indian worker died on June 9 after he was severely burned in an explosion aboard a vessel being painted at a nearby shipyard.
No explosion took place Thursday on board the ship that was being converted into a seismic survey vessel at DryDocks World Singapore, the report said.
The Manpower Ministry has issued a stop-work order on the vessel. The seven men were believed to have been working with heat or flame.
Experts told the newspaper that hazardous fumes or a lack of oxygen could have led to their collapse. They were working in a confined area.
One possibility raised was an accidental release of carbon dioxide, displacing the oxygen.
There were nine deaths last year in Singapore's shipbuilding and ship-repair industry.