A SOUTH Korean fighter jet with two crew aboard crashed into the sea, the defence ministry and the Air Force confirmed.
"One F-15K jet, which left a Daegu air base, suddenly disappeared from the Air Force radar while carrying out a night-time mission," the ministry and the Air Force said in a statement.
Wreckage was discovered in the Sea of Japan (East Sea) 350km southeast of Seoul, the statement said.
The fate of the two missing airmen was unknown and a navy search operation was continuing.
The pair has been identified only as veteran pilot Major Kim, 36, and 32-year-old Captain Lee.
Major Kim has accumulated more than 1000 flight hours in the last decade, the statement carried by Yonhap news agency said.
Seoul has placed an order for 40 F-15Ks from Boeing for delivery by 2008 for $US3.7 billion ($5.01 billion), and a first consignment of four is already being used by the South Korean Air Force.
The missing jet was one of that first batch.
News reports last month said that South Korea planned to cut back its purchase of US-made F-15 fighter jets over the next several years from 80 planes to 60 due to budget constraints.
The F-15K, built by US aviation giant Boeing, is one of the most sophisticated warplanes ever built.