The soldier was killed and the three others were injured while jumping from a height of 400 metres (1,300 feet) early Tuesday, Bulgarian army spokeswoman Violina Valeva said.

The accident happened "during an airborne drop operation", the Bulgarian defence ministry said.

The Canadian military confirmed that Patrick Labrie had died and said an investigation has been launched.

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

Three US paratroopers suffered arm and leg fractures, the BNT public television said, without giving details.

"The airborne jump was immediately suspended in order to attend to the paratrooper. All casualties were immediately transported to local hospitals where one was pronounced dead, one was admitted for further evaluation, and two were treated and released," US Army Europe said in a statement.

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

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