One soldier was killed and three others injured jumping from a height of 400 metres (1,300 feet) late Monday, according to Bulgarian army spokeswoman Violina Valeva.

The accident happened "during an airborne drop operation" at the so-called "Swift Response" exercise, the defence ministry said.

Dnevnik newspaper's web edition reported that one Canadian paratrooper died and another Canadian and two US soldiers were hospitalised with non-critical injuries.

The dead man's parachute failed to open at Cheshnegirovo airfield near Plovdiv, Bulgaria's second-largest city, the local trafficnews.bg website said.

Three other paratroopers who were close by suffered arm and leg fractures, the website added.

"Swift Response" -- a US Army Europe-directed multinational exercise -- is taking place June 11-24 in Bulgaria, Croatia and Romania with 5,600 soldiers from eight countries, including 1,495 paratroopers, involved.

The annual drills test deployment of high-readiness forces into a designated area while advancing airborne interoperability among NATO allies.