HANOI - Four Vietnamese military personnel were killed when an air force helicopter crashed during an exercise on Wednesday, the army said, in the second such accident in seven months.

The United States-made UH-1 helicopter went down in a suburb of Ho Chi Minh City and all those on board were killed, said Lieutenant-General Vo Van Tuan, deputy chief of the staff of the army.

"We are investigating the cause of the crash," Tuan told Reuters, adding that no civilians were affected.

Twenty Vietnamese military were killed in July when a Soviet-era MI-171 helicopter went down during a training flight outside the capital Hanoi. Sixteen died instantly and four succumbed to their injuries.

The UH-1, or Huey, as the model is better known, is one of the most flown helicopters in the world and was first put to use overseas in the 1960s by the United States during the Vietnam War. It has since been widely used by armed forces across Southeast Asia.