The incident involving an F-16 Fighting Falcon assigned to the 8th Fighter Wing happened early Monday over the Yellow Sea, the US Air Force said in a statement.

"The pilot ejected the aircraft. The pilot has been recovered by Republic of Korea Maritime Forces, awake and in stable condition," the statement said, adding he would be evaluated further back at base.

"We are grateful for the safe recovery of our Airman by our ROK Allies and that the pilot is in good condition," said Colonel Matthew C. Gaetke, the 8th fighter wing's commander, according to the statement.

The USAF said that the cause of the in-flight emergency was unknown.

"The incident will be thoroughly investigated," it said, adding that the name of the pilot would not be released, nor would further details about his condition.

South Korea's defence ministry declined to comment.

In May, a US F-16 jet crashed during a routine training exercise in a farming area south of Seoul. The pilot ejected safely and the accident caused no other casualties.

Washington is Seoul's most important security ally and stations about 28,500 troops in South Korea to help protect it from the nuclear-armed North.

In neighbouring Japan, the US military announced last week that it was grounding its fleet of V-22 Osprey tilt-rotor aircraft following a deadly crash that killed eight US airmen.