A Boeing 737 passenger plane carrying 62 people is believed to have crashed into the sea shortly after take-off from Indonesia's capital Jakarta, reports BBC.
The Sriwijaya Air plane lost contact four minutes into its journey to Pontianak in West Kalimantan province.
Flight tracking website Flightradar24.com said the aircraft lost more than 3,000m (10,000ft) in altitude in less than a minute.
Witnesses said they had seen and heard at least one explosion.
Fisherman Solihin, who goes by one name, told the BBC Indonesian service he had witnessed a crash and his captain decided to return to land.
"The plane fell like lightning into the sea and exploded in the water," he said.
"It was pretty close to us, the shards of a kind of plywood almost hit my ship."
A number of residents of an island near where the plane disappeared told the BBC they had found objects they thought were from the plane.
The transport ministry said search and rescue efforts were under way.
The Indonesian navy has been deployed to look for the aircraft, reports say. Navy official Abdul Rasyid told Reuters news agency it had determined the plane's coordinates and ships had been deployed to the location.
The aircraft is not a 737 Max, the Boeing model that was grounded from March 2019 until last December following two deadly crashes.
Last contact with the plane, with the call sign SJY182, was made at 14:40 local time (07:40 GMT), according to the transport ministry. The usual flight time to Pontianak, in the west of the island of Borneo, is 90 minutes.
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.
According to registration details, the plane is a 26-year-old Boeing 737-500.
Sriwijaya Air, a local budget airline which flies to Indonesian and other South-East Asian destinations, said it was still gathering information about the flight.
In October 2018, an Indonesian Lion Air flight plunged into the sea about 12 minutes after take-off from Jakarta, killing 189 people.
That crash 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.
The BBC's Jerome Wirawan in Jakarta says the latest events will bring up difficult questions and emotions in Indonesia, whose airline industry has faced intense scrutiny since the Lion Air crash.