A Russia-flagged fishing boat caught fire in waters off South Korea's southeastern city of Ulsan early Friday, with 21 sailors rescued and four others still missing.

According to the Ulsan Coast Guard Station, a 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.

After confirming the fire through an emergency position indicating radio beacon, the Coast Guard dispatched patrol and rescue vessels to the scene, while seeking assistance from nearby fishing boats.

Authorities have rescued 21 of the 25 Russian sailors, but four remain unaccounted for. Two out of the 21 that were rescued suffered minor burns and the rest are reportedly in good health.

The Air Force contributed to the efforts with flares to increase nighttime visibility while the Navy sent vessels to assist in the search and rescue.