Indonesian Navy divers pull a part of an airplane out of the water during a search operation for the Sriwijaya Air passenger jet that crashed into the sea near Jakarta, Indonesia, on Jan 10, 2021. (ACHMAD IBRAHIM / AP)

Chinese President Xi Jinping on Monday sent a message of sympathy to Indonesian President Joko Widodo over a crash of a passenger plane in Indonesia on Saturday.

Xi said in the message that he was shocked to learn about the accident. On behalf of the Chinese government and people, Xi expressed grief for the victims of the crash and offered his deep condolences to their families and loved ones.

A Boeing 737-500 of Sriwijaya Air crashed into the sea off Indonesia with 62 people aboard minutes after taking off from Indonesia's capital, Jakarta, during heavy rain on Saturday. It was en route to Pontianak in West Kalimantan.

More than a dozen helicopters, 53 navy ships, 20 boats, and 2,600 rescue personnel have been searching since Sunday. They have found parts of the plane in the water at a depth of 23 meters, leading rescuers to continue searching the area.

A Boeing 737-500 of Sriwijaya Air crashed into the sea off Indonesia with 62 people aboard minutes after taking off from Indonesia's capital, Jakarta, during heavy rain on Saturday

The search for the black boxes of the crashed plane intensified on Monday to boost the investigation into what caused the plane to nose-dive at high velocity into the Java Sea. The search so far has yielded plane parts and human remains but no sign of survivors.

Authorities said signals from the boxes containing the cockpit voice and flight data recorders were detected between Lancang and Laki islands in the Thousand Island chain just north of Jakarta's coast.

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Accident details

Officials said they have marked a location where the sounds were being emitted from the black boxes, which detached from the tail of the aircraft when it plummeted into the sea.

The cockpit voice recorder holds conversations between pilots, and the data recorder tracks electronic information such as airspeed, altitude and vertical acceleration.

When found, they will be transported to port and handed to the National Transportation Safety Committee overseeing the probe.

Television footages showed the landing gear, wheels and a jet engine among the parts found. Other rescuers brought a dozen body bags containing human remains to a police hospital in eastern Jakarta for the identification process.

National Search and Rescue Agency chief Bagus Puruhito said divers using a high-tech "ping locator" equipment were looking for an identified target beneath 20 meters of seabed mud.

The transport committee's chairman, Soerjanto Tjahjono, said the black boxes could provide valuable information to investigators.

Once the devices are found and taken to the investigators' facility, it will take three to five days to dry and clean the devices and to download its data, Tjahjono said, while ruling out a possible midair breakup after seeing the condition of the wreckage found by searchers.

Singapore's Transport Safety Investigation Bureau will help the committee in searching for the black boxes and the United States' National Transportation Safety Board will join in investigating the crash.

READ MORE: Indonesia's aviation safety under fresh spotlight after plane crash

Agencies contributed to this story.