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.
A passenger list of 36 names posted at Halim airport showed that most of the people on board were Indonesian airline and aviation representatives, as well as five journalists and a representative of French aircraft engine maker SNECMA, with a Vietnamese name.
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.
Prakoso said a team of around 200 police, military and rescue workers were headed in vehicles and on foot towards Salak mountain where the plane went missing.
"A helicopter was sent earlier, but it returned because it was getting dark and raining. We will continue searching by air again first thing tomorrow," he said.
The sprawling Indonesian archipelago relies heavily on air transport and has a poor aviation safety record.
The official Antara news agency said that domestic Indonesian carrier Kartika Airlines and regional Sky Aviation had both placed orders for the Sukhoi Superjet.
The Sukhoi Superjet 100 is a new passenger plane built by Russia in a bid to lift its civil aviation industry from a post-Soviet crisis.
The Superjet only made its first commercial flight last year and if a major accident is confirmed it would be the first disaster to involve the aircraft, which is made by legendary Russian planemaker Sukhoi.
The plane is crucial to Russia's hopes of becoming a major player in the modern aviation market and improving its image in an industry scarred by frequent crashes of ageing Soviet-era jets.
The mid-range Superjet airliner is designed to carry up to 98 passengers and is a direct rival of similar aircraft produced by Brazil's Embraer and Canada's Bombardier.
So far it is being flown by two airlines, Russia's Aeroflot and Armenia's Armavia, although orders have been confirmed with several more. Its first commercial flight was operated by Armavia in April 2011 and Aeroflot followed later that year.
The demonstration flight in Indonesia was part of an Asian roadshow to promote the aircraft that started May 3 and earlier took in Kazakhstan and Pakistan. It was due to go on to Laos and Vietnam.
The Superjet project is a joint venture between Sukhoi and Italy's Alenia Aeronautica, which is part of the aerospace and defence giant Finmeccanica.
The plane has experienced problems while being flown by Russian flag carrier Aeroflot, which has been under heavy pressure from the government to add more Russian planes to its fleet.
In March, a plane had to cut short a scheduled flight after it encountered problems with its undercarriage. Aeroflot's first Superjet plane also spent several weeks grounded upon delivery because of an air conditioning problem.© ANP/AFP