A Chinese trawler Reuters

Three Chinese fishermen were killed on Thursday after their fleeing boat burst into flames as the South Korean coastguard men threw flash grenades while trying to apprehend them for illegal fishing, a South Korean official said.

Authorities in the South Korean port city of Mokpo said the three fishermen were believed to have suffocated to death, but the investigators are looking into the matter to find out the exact cause.

Illegal fishing has always been an important factor in disputes between China and US ally South Korea and it has been dampening the relations between the two countries. Both the countries also share concern over North Korea's nuclear weapon and missile programmes.

An official, who is not authorised to speak with media and declined to be identified, told Reuters over phone that the fire broke out in the trawler's steering room.

The official also added that the South Korean authorities were questioning the 14 surviving crew and coastguard members involved in the operation.

However, when China's foreign ministry in Beijing was asked about the incident it was not available for any immediate comment. Also, a Chinese embassy official in Seoul could not be reached for comment.

The Chinese boats are been regularly chased by the South Korean coastguard vessels for illegal fishing. Such chasing has resulted into violent confrontations in the past.

The official also said the authorities would bring the Chinese boat which was caught fishing in waters off the southwest of the peninsula, about 70 km (43 miles) southwest of Hongdo Island, to a South Korean port later on Friday.

The South Korean coastguard and fishermen were frustrated with incursions by Chinese boats in defiance of coastguard warnings. They had captured two Chinese boats earlier and handed them over to authorities.

To tackle the problem of illegal fishing, South Korea and the United Nations Command, which oversees the Korean War armistice, launched a joint operation in June. The operation was started to keep the Chinese fishing vessels away from illegal fishing off the west coast of South Korea.