South Korea's Coast Guard said Friday it was investigating the death of three Chinese fishermen in a fire on their vessel after it was boarded by a coastguard patrol for fishing in Korean waters.

Disputes over illegal fishing have dogged relations between South Korea and China for years, and there have been numerous incidents of violent clashes between the coast guard and Chinese crew members.

A coastguard spokesman in the southwestern port of Mokpo said the Chinese boat had been spotted fishing Thursday in the waters of South Korea's exclusive economic zone, and was boarded after ignoring commands to stop.

Coast guard personnel then threw stun grenades into the wheelhouse where the crew had barricaded themselves inside and were continuing to pilot the boat.

The wheelhouse structure caught fire and three of the Chinese crew died in the ensuing blaze -- most likely due to smoke inhalation.

"The remaining 13 crew members and the skipper are in custody and being questioned. A forensic team is now ready to board the ship for inspection," the spokesman told AFP.

An autopsy had been ordered on the three dead fishermen to confirm the precise cause of death, he added.

In Beijing, the foreign ministry said consular officials had been sent to Mokpo and a request made for the South Korean authorities to preserve the bodies of those who died.

China called on South Korea to "conduct an all-round objective and unbiased investigation into the issue," ministry spokesman Geng Shuang told reporters.

Seoul has been asking Beijing to take a tougher stand on Chinese vessels that have been entering South Korean waters in increasing numbers to sate growing demand at home for fresh seafood.

Small, wooden Chinese ships were once tolerated in an area where the top priority has always been guarding against potential incursions from North Korea.

But in recent years, the small boats have given way to larger steel trawlers who engage in bottom trawling - dragging a large, weighted net across the sea floor that sweeps up everything in its path.

Around 2,200 Chinese vessels have been stopped and fined by South Korea for illegal fishing in the past four years, and the number of arrested fishermen jumped from two in 2010 to 66 in 2013.