A Russian jet with more than 40 people on board is missing in a mountainous area south of the Indonesian capital, Jakarta.
The plane was on a demonstration flight and disappeared from 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.
Around 200 police, military and rescue workers were headed in vehicles and on foot to 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 added.
The plane took off at 2pm (8am Irish time) from east Jakarta's Halim Perdanakusuma airport.
"At 2:50pm it dropped from 10,000 feet (3,048 metres) to 6,000 feet," the rescue agency said in a text message to AFP.
Reports of the exact number on board varied slightly, with Mr Prakoso saying it was carrying 46 people.
Russia's RIA Novosti news agency reported 36 non-Russians on board and eight Russians, four of them crew and the others Sukhoi company representatives.
Herry Bakti, head of Indonesia's transport ministry's aviation division, said Superjet was on the second of two demonstration flights, and those on board were invited guests.
"It was just going to briefly circle the area and land back at Halim airport," he said.
The Superjet 100 is a new passenger aircraft built by Russian planemaker Sukhoi in an attempt to lift the country's civil aviation industry from a post-Soviet crisis.
It 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.
