SEOUL (Reuters) - Three Chinese fishermen were killed on Thursday in a fire that broke out on their boat when South Korean coastguard men trying to apprehend them for illegal fishing threw flash grenades into a room they were hiding in, a South Korean official said.

Disputes over illegal fishing are an irritant in relations between China and U.S. ally South Korea, even as their economic relations grow close. They also share concern about North Korea’s nuclear weapon and missile programs.

The three men were believed to have suffocated, a coastguard official in the South Korean port city of Mokpo said, adding that the incident was being investigated.

The fire broke out in the boat’s steering room, the official, who is not authorized to speak with media and declined to be identified, told Reuters by telephone.

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

China’s Foreign Ministry said it had lodged a protest with Seoul about the incident.

Ministry spokesman Geng Shuang told a daily news briefing Beijing was also urging South Korea to hold a “comprehensive and objective” investigation into the incident, along with China.

South Korean coastguard vessels regularly chase Chinese boats for fishing illegally and violent confrontations have occurred in the past.

The Chinese boat, caught fishing off the southwest of the peninsula, about 70 km (43 miles) southwest of Hongdo Island, would be brought in to a South Korean port later on Friday, the coastguard official said.

In June, South Korea and the United Nations Command, which oversees the Korean War armistice, launched a joint operation to keep Chinese fishing vessels from operating illegally off South Korea’s west coast.

That came after South Korean fishermen, frustrated with incursions by Chinese boats in defiance of coastguard warnings, impounded two Chinese trawlers and handed them over to authorities.