The Hong Kong Maritime Rescue Coordination Center said the tanker was 300 kilometers (186 miles) east of Hong Kong when it caught fire as a result of an explosion. The fire was extinguished, according to state-run Radio Television Hong Kong.
One crew member was reported to have died and four others were in serious condition, RTHK reported.
The cause of the explosion was not immediately clear.
The casualties reportedly included crew members from Indonesia and Burma (also known as Myanmar).
According to ship tracking apps, the 5,500-ton oil and chemical tanker was on its way to Taiwan. The 120-meter (393-foot) long tanker is 22 years old.