An air crash that killed four Vietnamese soldiers on Wednesday morning happened after the crew failed to fix a technical error, a colonel said.
Colonel Le Van Hanh, the head of the division's Communist Party Unit in the Air Force, said there was a “significant breakdown.”
"They tried to fix it but they did not succeed."
Hanh said he could not disclose the details of the problem since the investigation remains underway.
The Vietnam War era UH-1 Iroquois went down minutes after taking off at Tan Son Nhat Airport at roughly 7AM during a training flight bound for neighboring Tay Ninh Province.
The wreckage has remained at the crash site, a mangrove forest in suburban Binh Chanh District.
An eyewitness said he was rowing a boat in the forest and a strong gust blew off his hat.
“Just when I bent to pick it up, I heard a loud bang. I checked my watch and it was 7:17AM.”
Hanh said it took them around two and a half hours to find the aircraft in the "complicated" terrain.
Officials suggested that the crew made every effort to avoid going down over roads and houses.
No civilian casualties have been reported thus far.
Locals said the area was very smoky at the time and many vehicles had to switch on their headlights.
A UH1 helicopter of Vietnam Air Force in a file photo taken in 2014
Four ambulances carried the bodies to a funeral home overseen by the Ministry of Defense in Ho Chi Minh City that afternoon.
Investigators have identified them as Tran Van Duc, a regiment deputy chief of staff and piloting the flight, Do Van Chinh, a technician, and two co-pilots Le Hong Quan and Nguyen Viet Cuong. Cuong was being trained to become a pilot.
Questioned about the aircraft's age, Lieutenant General Vo Van Tuan, Deputy Chief of Staff of the People’s Arm, said it wouldn't have been allowed to take off if it hadn't met the requisite flight requirements.
Vietnam seized about 50 UH-1, better known as "Hueys," from the US-backed army of South Vietnam after the country's reunification in 1975. Local media reported last year that 14 of them were still operational.
Last July, a Russian-made Mi-171 chopper crashed on the outskirts of Hanoi during a training exercise, killing 20 soldiers.
Only one man survived with severe injuries.