The Sriwijaya Air passenger plane departed from Jakarta airport at 14:36 local time on Saturday.
At 14:40, the last contact with the plane was recorded according to the transport ministry.
The usual flight time to Pontianak, in West Kalimantan province in the west of the island of Borneo, is 90 minutes.
There was no distress signal from the aircraft according to the head of national search and rescue agency Air Marshal Bagus Puruhito.
It is thought to have dropped more than 3,000m (10,000ft) in less than a minute, according to flight tracking website Flightradar24.com.
Witnesses said they had seen and heard at least one explosion.
The Sriwijaya aircraft disappeared 4 minutes after takeoff.
At 14:40, the last contact with the plane was recorded according to the transport ministry.
The usual flight time to Pontianak, in West Kalimantan province in the west of the island of Borneo, is 90 minutes.
There was no distress signal from the aircraft according to the head of national search and rescue agency Air Marshal Bagus Puruhito.
It is thought to have dropped more than 3,000m (10,000ft) in less than a minute, according to flight tracking website Flightradar24.com.
Witnesses said they had seen and heard at least one explosion.
The Sriwijaya aircraft disappeared 4 minutes after takeoff.
There were thought to be 50 passengers – including seven children and three babies – and 12 crew on board, though the plane has a capacity of 130. Everyone on board was Indonesian, officials say.
“I have four family members on the flight – my wife and my three children,” Yaman Zai told reporters through tears.
“[My wife] sent me a picture of the baby today… How could my heart not be torn into pieces?”
About the plane
According to registration details, the plane was a 26-year-old Boeing 737-500.
It was in good condition, Sriwijaya Air chief executive Jefferson Irwin Jauwena told reporters. Take-off had been delayed for 30 minutes due to heavy rain, he said.
Sriwijaya Air, founded in 2003, is a local budget airline which flies to Indonesian and other South-East Asian destinations.
The plane went missing about 20km (12 miles) north of the capital Jakarta, not far from where another flight crashed in October 2018.
A total of 189 died when an Indonesian Lion Air flight plunged into the sea about 12 minutes after take-off from the city.
That disaster was blamed on a series of failures in the plane’s design, but also faults by the airline and the pilots.
It was one of two crashes that led regulators to pull the Boeing 737 Max from service. The model resumed passenger flights in December after a systems overhaul.
