The training area where the US helicopter crashed is about 15 miles from the border with North Korea.
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.
The South Korean media is reporting that the injured from the US military helicopter crash near the North Korean border are being taken to the hospital at Yongsan army base.
A statement is expected imminently from the US Marines following reports that an American military helicopter crashed near the North Korean border.
French news agency AFP said a CH-53 Sea Stallion helicopter was taking part in a joint drill with South Korean allies when the incident occurred but reported no casualties.
The Yonhap news agency said the helicopter came down in Cheorwon province, which borders North Korea.