A small wooden bridge collapsed in the border town of Hwacheon on Thursday, injuring 21 soldiers who were undergoing midnight training, military authorities said.

The bridge on a mountain trail in Hwacheon, Gangwon Province, measured 15 meters in length and 2.5 m in width, caved in around 1:30 a.m. while the soldiers were marching over the bridge as part of a nightly tactical training, according to the military.

A total of 21 servicemen fell 3 meters into a ravine, sustaining various injuries, the military said.

All of the soldiers, including two 23-year-olds surnamed Yoo and Lim, were rushed to a nearby private hospital and a military hospital, but none of them were in serious condition, according to the authorities.

"The unexpected accident took place even though geographic reconnaissance and a safety check-up were carried out ahead of the tactical training," one military official said.

A military investigation is now underway to find the exact cause of the collapse of the bridge, which has been frequently used by civilian hikers.

"The bridge seems to have succumbed to the heavy weight of (the marching soldiers) given that its welds came apart," a Hwacheon county official said. "An investigation is also underway to see if the seven-month-old wooden bridge was constructed poorly." (Yonhap)