One American Marine was killed and rescue teams were searching for five others missing after two Marine Corps aircraft collided in mid-air and crashed into the sea off the coast of Japan during a refueling exercise, officials said on Thursday.

Japanese and American officials said they had so far found two of the seven Marines who had been aboard the aircraft, an F/A-18 Hornet fighter jet and a KC-130 Hercules.

"One of the recovered Marines is in fair condition and the other has been declared deceased by competent medical personnel," a US Marine Corps statement said.

Takeshi Iwaya, the Japanese Defence minister, said: "US military and the Japanese Self-Defense planes and vessels are searching for those still missing... I hope all the members will be rescued safely as soon as possible."

The marines were conducting "regularly scheduled training" when the crash occurred around 2:00 am local time, the 3rd Marine Expeditionary Force said in a statement.