Vietnam's prime minister ordered the military to launch a rescue mission after a helicopter went missing on Tuesday during a training flight, the government said, in the latest in a series of aviation setbacks.

The Airbus EC 130 T2 helicopter was carrying an instructor and two trainee pilots when it disappeared off radar screens in southern Ba Ria-Vung Tau province, the government said on its website.

The defense ministry issued a statement but did not indicate whether those aboard were military or civilian.

Prime Minister Nguyen Xuan Phuc instructed the military to find the victims and review procedures to ensure safety and asked rescue units and provincial staff to help.

The incident comes after a succession of deadly accidents involving military planes and helicopters, at a time when Vietnam is adding to its shopping list as part of its biggest military buildup since the American war in Vietnam.

Defense experts and arms firms say the country is on the lookout for anything from fighter jets and helicopters to surveillance planes, radar and surface-to-air missiles.

The air force lost two fighter jets, in June and August. After the earlier crash, a CASA 212 coastguard plane hunting for the pilot and wreckage of a Russian-made Sukhoi SU-30 went down in bad weather, killing all nine people aboard.

It has also had two helicopters crashes since July 2014, killing more than 20 people.