Three Chinese fishermen were killed Thursday after a fire broke out on their boat during an inspection by the Korean coast guard, officials said.

The 102-ton Chinese vessel "S" was sailing in South Korean waters about 70 kilometers southwest of Hong Island near the southwestern city of Mokpo at 9:45 a.m. when coast guard officers ordered it to stop.

Illegal fishing by Chinese fishermen in South Korean waters has been a constant source of trouble between the two countries, sometimes leading to violent clashes.

The officers had intended to check whether the boat had a license to fish inside South Korea's Exclusive Economic Zone and had observed the fishing quota, but the vessel kept sailing, according to the coast guard officials.

Fourteen coast guard officers boarded the boat to carry out the inspection, but the Chinese crew locked the steering house and engine room. The fire started after the officers broke the windows of the steering house and threw three flare and sound bombs inside, the officials said.

The officers rescued 14 of the 17 crew members, but the other three were found laying unconscious inside the engine room around noon. They were moved to a South Korean patrol boat for emergency treatment but were declared dead by a Korean doctor at 3:46 p.m.

"The exact cause of the fire has not been determined yet," said one coast guard official who spoke on the condition of anonymity.

"We may request an autopsy to determine the cause of death."

Chinese consular officials visited the coast guard office in Mokpo to demand a thorough investigation. (Yonhap)