SEOUL (AFP) - A US military helicopter taking part in a joint South Korea-US drill crashed on Tuesday near the North Korean border with 21 personnel on board. There were no fatalities.

A statement put out by the US Forces in Korea said the CH-53 Super Stallion helicopter had "executed a hard landing" while on a routine flight operation in Cheolwon county, which touches on the border with North Korea.

The precise cause of the accident was not immediately known, the statement said, adding that a "comprehensive investigation" had been ordered.

All five crew members and 16 other service personnel were taken to a US military hospital in Seoul. Fifteen were subsequently released while six remained hospitalised "in stable condition", the statement said.

Pictures from the site taken by the Yonhap news agency showed firefighters using foam to douse the burned out wreck of the helicopter which had clearly burst into flames after crash landing.

The helicopter was taking part in an annual South Korea-US joint military exercise called "Foal Eagle."

The accident occurred at a time of heightened military tensions on the Korean peninsula, partly due to North Korea's anger over "Foal Eagle", which it sees as an invasion rehearsal.

There are 28,500 US military personnel permanently stationed in South Korea.

The crew of the crashed helicopter belonged to the 31st Marine Expeditionary Unit stationed in Okinawa, Japan.