The fourth day of the search for a missing Royal Cambodian Air Force helicopter has so far shown no results, as searchers continue to scour the rugged terrain of the Cardamom Mountains.

The aircraft was reportedly on a routine search-and-rescue training exercise on the afternoon of July 12 when it disappeared during bad weather. An air force statement explained that the authorities are concentrating their efforts within an isolated 50sq km of wilderness.

It has been announced that the Chinese-built Harbin Z-9 was carrying two air force officers, Sun Pla and Kheng Chaiyuth.

The Z-9, a twin-engined medium multi-use helicopter, is one of 12 received by the Royal Cambodian Armed Forces in November 2013. They were acquired as part of a $195 million deal with China, inked in 2011.

One of the fleet was lost in an earlier crash near Phnom Penh’s Prey Sar prison in Dangkor district in July 2014. The accident killed five members of the armed forces and left one more of the crew severely injured.

The aircraft has been widely exported from China, with regional orders received from the militaries of Myanmar and Laos, as well as Pakistan and Bangladesh.

It has also proved popular in Africa, and is in service in Cameroon, Namibia and Zambia, among other nations.

Originally a license-built copy of the French Aerospatiale SA 365 Dauphin 2, which first flew in 1975, the model was upgraded by Chinese engineers through the introduction of more powerful engines and a more sophisticated avionics package.