The helicopter, part of a search team led by a controversial Myanmar tycoon, was carrying three people, including a Thai pilot, expedition organisers said.
"The chopper... lost contact (Saturday) while on their way to drop supplies to rescue teams searching for the two missing mountaineers," the Htoo Foundation, which deployed the helicopter, said in a statement.
Contact was lost with ground control shortly after it took off from Putao airport in northern Kachin state on Saturday afternoon, it added.
Two other helicopters from the foundation and a government Mi-17 helicopter are also searching for the missing aircraft "and soldiers, local police force and local people are cooperating", the statement said.