The US military claimed one of its helicopters carrying Marines executed a hard landing near the North Korean border "while conducting routine flight operations".
A statement said the US Marine Corp. CH-53E Super Stallion had 21 people on board including five crew members, all of whom were taken to hospital.
Fifteen people have been released from hospital and six are in a stable condition.
"A comprehensive investigation will take place to determine the facts and circumstances surrounding this incident", the statement added.
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