A helicopter crash in Sri Lanka has killed six military personnel, Reuters reports.

A Bell 212 helicopter had crashed into the Maduru Oya reservoir in central Sri Lanka, with a dozen armed forces personnel on board.

Six died after they were rescued and rushed to hospital, Sri Lanka Air Force spokesman Group Captain Eranda Geeganage said.

"The helicopter was assigned to conduct a grappling exercise at a passing-out parade. Four special forces personnel and two Air Force gunmen died of their injuries," Geeganage told Reuters.

But he declined to give details on the possible reason for the crash.