The South Korean military has launched a search and rescue operation in waters off the southeastern city of Ulsan for four missing crew members of a Russia-flagged fishing boat that caught fire in the night.

An official from the Joint Chiefs of Staff(JCS) said on Friday that the military has actively provided support on humanitarian grounds during times of emergency involving a foreign vessel in waters near South Korea’s maritime jurisdiction.

After the Coast Guard notified the military about the fire on a fishing vessel with 25 sailors at around 1:20 a.m., the JCS dispatched a nearby Aegis destroyer and a high-speed boat docked at the Busan port to assist in the search.

An Air Force plane was sent to release 42 flares to improve visibility, while the Navy deployed a patrol aircraft, rescue ship and high-speed boat, with initial efforts retrieving 21 of the sailors, with four still unaccounted for.

According to the Ulsan Coast Guard Station, the fire broke out aboard the 769-ton vessel at around 12:43 a.m. in waters some 52 kilometers southeast of Ulsan as it traveled to Russia after departing from Busan Thursday afternoon.