It was not immediately clear if there were any survivors and the United Nations could not confirm the number of people on board.
A police officer in Kathmandu said 10 bodies had been recovered.
The chopper was flying UN officials on an arms monitoring mission at the camps of former Maoist rebels who joined the government after a landmark peace deal in 2006.
"Police have already reached the site," Ramesh Mahat, an airport official, said. "It is raining there."
The helicopter went missing on its way to Kathmandu from Sindhuli, about 65 kilometres from the Nepali capital. (Reuters)
Chopper carrying UN staff crashes in Nepal, 10 dead