A Sriwijaya Air plane is suspected of having crashed minutes after take-off from the Indonesian capital, Jakarta, on Saturday, the authorities said.
Tracking showed the path of Flight SJ182 ending off the coast slightly north of Jakarta after the plane abruptly lost altitude.
Rescuers are looking for the plane, which lost contact after taking off with more than 60 people on board, an official of the Basarnas search and rescue agency said.
It has not yet been confirmed that debris found in the sea came from the Sriwijaya flight.
The Indonesian navy said it had determined the co-ordinates of the downed jet.
A statement released by the budget airline said 56 passengers and six crew members were onboard the Boeing 737-500, which was on a 90-minute flight from Jakarta to Pontianak, the capital of West Kalimantan province on Borneo island.
The authorities started search and rescue operations for the plane after contact was lost at 2.40pm local time, transport ministry spokesman Adita Irawati said.
According to flight-tracking website FlightRadar24, the aircraft appeared to have plunged 3,000 metres in less than a minute, about four minutes after take-off.
Sriwijaya Air has about 19 Boeing jets, which fly to destinations in Indonesia and South-East Asia.
In October 2018, 189 people were killed when a Lion Air Boeing 737 MAX jet slammed into the Java Sea about 12 minutes after take-off from Jakarta on a routine one-hour flight.
Boeing was fined $2.5 billion after that crash and a subsequent fatal flight in Ethiopia over claims it defrauded regulators overseeing the 737 MAX model, which was grounded worldwide following the two deadly crashes.
But Indonesia’s aviation sector has long suffered from a reputation for poor safety, and its airlines were once banned from entering US and European airspace.
In 2014, an AirAsia plane crashed with the loss of 162 lives.
Domestic investigators’ final report showed a chronically faulty component in a rudder control system, poor maintenance and the pilots’ inadequate response were major factors in what was supposed to be a routine flight from the Indonesian city of Surabaya to Singapore.