The apes were seen by a survey delegation comprising staff from the Quang Binh Forest Rangers Unit, the Viet Nature Conversation Centre and the Vietnam Union of Science and Technology.
The 20,000-hectare Dong Chau - Nuoc Trong area has been suggested by the Ministry of Agriculture and Rural Development to become a nature reserve.
Experts say they noticed 58 packs of southern white-cheeked gibbons within the survey area of Nuoc Trong stream and two other packs outside the survey area.
They estimate that this area has the largest number of southern white-cheeked gibbons in the North Central region of Vietnam.
Whilst conducting their survey along the Ho Chi Minh Trail in Quang Binh province, experts also spotted nine herds of red-shanked douc monkeys with a total of between 98 and 108 individuals.
According to the Quang Binh Forest Rangers Unit, the Dong Chau forest - Nuoc Trong stream area is a biologically diverse region home to a number of species listed as endangered and critically endangered such as the saola, the pangolin, giant muntjacs, southern white-cheeked gibbons and red-shanked douc monkeys.
The southern white-cheeked gibbon, whose scientific name is Nomascus siki, is found in southern Laos and north-central Vietnam.
The species has been listed as endangered by the International Union for Conservation of Nature primarily due to logging, agricultural encroachment and hunting.
