JAKARTA:Indonesia detected signals on Sunday that could come from a flight recorder of a Sriwijaya Air jet that crashed into the sea soon after taking off from the capital Jakarta, as human body parts and suspected pieces of the plane were retrieved.

The Boeing 737-500 with 62 passengers and crew was headed to Pontianak in West Kalimantan before it disappeared on Saturday from radar screens four minutes after takeoff.

The crash is the first major airline incident in Indonesia since the crash of a Lion Air Boeing 737 Max in 2018 that killed all 189 passengers and crew. That plane also plunged into the Java Sea soon after takeoff from Soekarno-Hatta International Airport.

"We have detected signals in two points. This could be the black box," Bagus Puruhito, chief of Indonesia's search and rescue agency, told reporters aboard a military ship.

Indonesian Navy official Wahyudin Arif told iNEWS they had found suspected pieces of the plane fuselage of about one metre (three feet) in length, part of a tyre and human body parts.

Media reports said body parts had been taken to a police hospital for identification.

Flight SJ 182 had 12 crew and 50 passengers on board, all Indonesians and including seven children and three babies.

"I am optimistic we will find (the plane) soon," Henri Alfiandi, an assistant to the chief of staff of the Indonesian Air Force, told a news briefing.

President Joko Widodo, speaking at the palace in Bogor, expressed "deep condolences" over the disaster and urged the public to pray the missing people could be found.

The search is focussing on the outer ring of the Laki and Lancang islands off the Jakarta coast. The sea in this area is about 20 to 23 metres (65-75 feet) deep.

"WE FEEL POWERLESS"

With a massive search and rescue mission underway, there were no immediate clues on what may have caused the jet's sudden descent. Safety experts stress most air accidents are caused by a cocktail of factors that can take months to establish.

Tracking service Flightradar24 said the aircraft took off at 2:36 p.m. local time (0736 GMT) and climbed to reach 10,900 feet within four minutes. It then began a steep descent and stopped transmitting data 21 seconds later.

A transport ministry spokeswoman said air traffic control had asked the pilot why the plane was heading northwest instead of on its expected flight path just seconds before it disappeared.

The pilots had decades of experience between them with the flight captain reported to be a former air force pilot and his co-pilot had been at Sriwijaya Air since 2013, according to his Linkedin profile.

The Sriwijaya Air plane was a nearly 27-year-old Boeing 737-500, much older than Boeing's problem-plagued 737 MAX model. Older 737 models are widely flown and do not have the stall-prevention system implicated in the MAX safety crisis.

"We are in contact with our airline customer and stand ready to support them during this difficult time," Boeing said in a statement. "Our thoughts are with the crew, passengers, and their families."

Distraught relatives waited in Pontianak about 740 km (460 miles) from Jakarta for news of their loved ones. At Jakarta's main airport a crisis centre was set up for families.

"We feel powerless, we can only wait and hope to get any information soon," Irfan, who had five relatives on the flight, told reporters.

Founded in 2003, Jakarta-based Sriwijaya Air group flies largely within Indonesia's sprawling archipelago. The budget airline has had a solid safety record, with no onboard casualties in four incidents recorded on the Aviation Safety Network database.

In 2007, the European Union banned all Indonesian airlines following a series of crashes and reports of deteriorating oversight and maintenance since deregulation in the late 1990s. The restrictions were fully lifted in 2018.