The passenger plane of the Indonesian Sriwijaya Air is believed to have crashed into the ocean immediately after taking off from the international airport of the capital Jakarta.

This Boeing 737-500, which had 56 passengers and six crew members on board, was en route from Jakarta to Pontianak city, AAP reported. The contact with the aircraft was lost immediately after takeoff, and the flight radar was showing that it was gaining altitude, but the it disappeared.

The flight authorities say they have not established contact with the plane since 2:40 p.m. local time.

Flight SJ182 departed from Jakarta's Soekarno-Hatta International Airport at 2:14 p.m., and it was scheduled to arrive in Pontianak at 3:30 p.m.

Of the 56 passengers on board, 46 are adults, seven are children, and three are newborns. The crew consisted of two pilots and four flight attendants.

Authorities believe it fell into the ocean off the coast of Jakarta.

Tracking service Flightradar24 said that about four minutes after taking off from Jakarta, the plane dropped more than 10,000 feet in one minute.

Indonesian fishermen have told authorities they had seen the plane drop into the ocean and photographed the wreckage of this ill-fated flight.