Civilian and army helicopters and a foot patrol scoured forest terrain in Nepal's eastern Taplejung district where the helicopter disappeared on Saturday, but abandoned the search at dusk after bad weather hampered efforts, rescue units said.
They are now set to resume the search early Sunday morning.
"There is a high possibility that the helicopter may have crashed," said Himesh Lal Karna, air traffic service manager at Kathmandu airport.
Passengers included Nepal's forest minister and his wife, the Finnish embassy's charge d'affaires and several senior members of the WWF conservation agency, including four Nepalis, the organisation said on its website.
Two Russian crew members were also on board, the WWF said.
No radio contact
The helicopter left Ghunsa village on Saturday morning but failed to arrive at Suketar village, its intended destination, a 20-minute flight away, the rescue unit said in a statement on Saturday.
The missing helicopter has not been in radio contact since it disappeared.
Although several passengers were carrying satellite phones, no contact had been made with the WWF office in Nepal, Trishna Gurung, a WWF official in Kathmandu, told AFP news agency.
The government minister, the WWF officials and journalists had flown to Taplejung to attend a function to mark the handing over of the Kanchanjunga conservation area project to the local community.