Vietnam's prime minister ordered the military to launch a rescue mission after a military helicopter went missing on Tuesday during flight training.
The government said on its website that the Airbus EC 130 T2 helicopter was carrying an instructor and two trainee pilots when it went off the radar in southern Ba Ria-Vung Tau province.
The defence ministry has also issued a statement regarding the missing helicopter. But the ministry did not say if those aboard were military personnel or civilians.
Prime Minister Nguyen Xuan Phuc asked the military to search for the victims and review procedures to ensure safety. He also asked the rescue units and provincial staff to help in the process.
This is the latest incident in a series of deadly aviation accidents that involved military aircraft.
The defence experts and arms firms say Vietnam is buying up fighter jets, helicopters, surveillance planes radars and surface-to-air missiles as it is working on its military buildup.
Earlier this year, a CASA 212 coastguard plane went hunting for the pilot and the wreckage of a Russian-made Sukhoi SU-30 but it went down due to bad weather resulting in the death of all the nine people onboard.
Apart from this, the air force lost two fighter jets this year in June and August respectively.