A Russian Sukhoi Superjet 100 on a demonstration flight with at least 44 people aboard went missing in a mountainous area south of the Indonesian capital Jakarta on Wednesday, officials said.
"The plane disappeared from the radar around the Bogor area. We are still looking for it and we are uncertain whether it crashed," said Gagah Prakoso, spokesman for the national search and rescue agency.
The plane took off at 2:00 pm (0700 GMT) from east Jakarta's Halim Perdanakusuma airport, which is used for some commercial and military flights, he said.
"At 2:50 pm it dropped from 10,000 feet (3,000 metres) to 6,000 feet," the agency added in a text message to AFP.
Transport ministry spokesman Bambang Ervan said the descent was intentional, and that the pilot was trying to head back to the Halim air field.
He said the flight went missing "in a mountainous area 10 nautical miles from Bogor military airbase."
The rescue agency spokesman said it was carrying 46 people but Dimitry Solodov, press attache at the Russian embassy in Jakarta, said he could confirm 44 on board, eight of them Russian airline crew members, and some journalists.
Herry Bakti, head of the transport ministry's aviation division, said the aircraft was on the second of two demonstration flights, and those on board were invited guests.
The Sukhoi Superjet, a mid-range airliner that carries up to 100 people, has been presented as Russia's contender to compete in the international civil aviation industry.
After its maiden flight in 2008, deliveries were delayed several times, but the planes are already in use with Russia's largest carrier Aeroflot and have been ordered by smaller airlines in Russia and abroad.
It claims to offer lower operating costs than rival aircraft made by Brazil's Embraer and Canada's Bombardier.
The official Antara news agency said that domestic Indonesian carrier Kartika Airlines and regional Sky Aviation had both placed orders for the Sukhoi Superjet.