Beijing dispatched a professional rescue ship at 9 pm on Saturday for a joint search mission with ROK helicopters, an official surnamed Zhang from the China Maritime Search and Rescue Center said in a telephone interview on Sunday.

A Chinese trawler (C) capsizes after a collision with an ROK Coast Guard ship in the early afternoon of Dec 18. [Photo/Agencies]

The ROK Coast Guard said there were dwindling hopes for the only missing man to be alive after the day-long search operation, considering the freezing conditions at sea, according to Yonhap.

Relevant organizations from the Chinese side have stepped up the pace of the investigation, including the evidence collecting and coordination with different counterparts from the ROK that have embarked on the case.

The ROK's Coast Guard vowed to report the incident to Beijing through diplomatic channels, but the Chinese Foreign Ministry has yet to comment on this issue.

The ROK accused the Chinese vessel of engaging in "illegal fishery", but China and the ROK had agreed on a fishery deal to regulate the working areas of fishing boats from both sides and commonly exploit ocean resources in an orderly and cooperative fashion.

So the Chinese vessel "should be working in line with relevant rules", the source said.

But the Chinese side "will look into the case seriously before making any prior judgments", the source added.

"The top priority is trying to find the missing fisherman."