Seoul, South Korea CNN  — 

A U.S. Marine helicopter participating in annual military exercises in South Korea made a hard landing Tuesday in a province that borders North Korea, the U.S. military said.

The 21 personnel on board the CH-53E Super Stallion helicopter, which was making routine flights as part of the exercises, were taken to a hospital at the United States’ Yongsan Garrison in South Korea, the military said in a statement.

Fifteen of those have since been released from the hospital, and the other six are in stable condition, the statement said.