A fisherman who was earlier quoted by BBC Indonesia as saying he believed the plane crashed near to his boat in the Java Sea has told Reuters that he and his two colleagues saw plane debris and fuel near their boat following an explosion around 30 metres away.

“We thought it was a bomb or a tsunami since after that we saw the big splash from the water after the explosion,” said the 22-year-old from Lancang island, named Solihin.  

“It was raining heavily and the weather was so bad. So it is difficult to see around clearly. But we can see the splash and a big wave after the sounds. We were very shocked and directly saw the plane debris and the fuel around our boat,” he said.

Air traffic control at Soekarno-Hatta International Airport had asked the pilot why the plane was heading northwest instead of on its expected flight path just seconds before it disappeared, a transport ministry spokesperson told Reuters.

The news agency reported that there were no immediate clues on what may have caused the sudden descent and safety experts stress most air accidents are caused by a cocktail of factors that can take months to establish.

All of the passengers and crew onboard the flight are Indonesian, the country's transport safety committee has said.

While the Indonesian Navy has pinpointed the site of the missing aircraft and ships have been sent there, according to a Navy official, authorities have not yet said whether they believe there are any survivors.

Military personnel have set up a crisis centre inside Jakarta airport:

Bambang Suryo Aji, the National Search and Rescue Agency's deputy head of operations and preparedness, said no radio beacon signal had been detected from the 26-year-old plane. 

He said his agency was investigating why the plane's emergency locator transmitter, or ELT, was not transmitting a signal that could confirm whether it had crashed.

“The satellite system owned by neighbouring Australia also did not pick up on the ELT signal from the missing plane,” Mr Aji said.

Four warships were among the dozen vessels deployed on the search-and-rescue mission centred between Lancang island and Laki island, just north of Jakarta, transportation minister Budi Karya Sumadi has said.

This area is where footage showed fishermen pulling what appeared to be plane debris from the water.

Bambang Suryo Aji, the National Search and Rescue Agency's deputy head of operations and preparedness, said the items had been handed over to the National Transportation Safety Committee for further investigation to determine whether they were from the missing plane.

The Associated Press reports that Sriwijaya Air has had a solid safety record until now, with no onboard casualties in four incidents recorded on the Aviation Safety Network database. 

However, a farmer was killed when a Boeing 737-200 left the runway in 2008 following a hydraulic problem.

Here’s footage of the devastating scenes at the airport in Pontianak, where the flight was due to land.

The missing plane was in good condition when it took off, the airline's chief executive Jefferson Irwin Jauwena told reporters.

He also said the flight was delayed for 30 minutes before it could take off due to heavy rain.

The coordinates of the missing plane have been found and given to all Navy vessels in the area, a Navy official has told reporters.

“The Indonesian Navy has determined the coordinates of a Sriwijaya Air plane that went missing after taking off from the capital Jakarta and ships have been deployed to the location,” Reuters reported Abdul Rasyid as saying.

“The coordinates have been found and have been given to all Navy vessels in the area,” he said.