The WWF global environmental group, which had organised the trip to the Taplejung district, 300 km from the capital Kathmandu, said the helicopter was carrying two Americans, a Finnish, a Canadian and an Australian national when it disappeared.

It said two Russians were part of the four-member crew.

"The helicopter carrying delegates who had gone to attend the Kanchenjunga Conservation Area handover at Taplejung area is missing," said a WWF statement.

"The flight left Ghunsa (village) this afternoon at 12 p.m. (0615 GMT) but did not arrive as per schedule back to Taplejung."

The WWF said junior forest minister Gopal Rai, his wife and three senior government officials were among the 20 passengers on board the Russian-made M1-17 helicopter, owned by local private company, Sri Airlines.

Airport officials coordinating the search and rescue operation said so far there was no trace of the missing aircraft.

"We fear an unfortunate incident may have occurred. We have sent two helicopters for search operations because we have reports that weather is improving," Ambar Bahadur Shakya, told Reuters.

He said two helicopters had earlier been dispatched to conduct an aerial search but bad weather had forced them to turn back. Two ground teams have also been deployed to the area, which is remote and mountainous.

There are more than a dozen private airlines in Nepal, which has a poor road network and some of the world's highest peaks, including Mount Everest. In 2002, a small passenger plane crashed in west Nepal killing 18 people, including 13 German nationals.