Chinese navy ships have arrived off Bali to help haul up a submarine that sank last month killing 53 crew, the Indonesian Navy said on May 4 as it geared up for a deep-sea salvage operation.

The help arrived after other foreign ships from Australia, Singapore and Malaysia left the archipelago, having assisted Indonesian authorities in finding the stricken vessel.

The KRI Nanggala 402 – one of five submarines in Indonesia's fleet – disappeared last month while it was scheduled to take part in live torpedo training exercises.

An underwater rescue vehicle supplied by neighbouring Singapore gave visual confirmation that the German-built sub was lying on the sea floor more than 800m deep, broken in three parts, confirming there was no hope of finding survivors.

Two Chinese salvage ships were on standby in waters off Bali, while a third was expected to arrive later on May 4, the Indonesian Navy said, adding that Chinese Navy officials in Bali were helping to examine data collected on the submarine.

All three salvage ships can dive up to 4,500m deep.

Indonesia's Navy said Beijing's ambassador to the country had offered the help to defence minister Prabowo Subianto.

"The offer was welcomed by the Indonesian government," it said in a statement on May 4.

A vessel from Indonesia's upstream oil and gas regulator task force SKK Migas, used for drilling operations, will also join the salvage operations. It has a crane with a capacity of 1,200 metric tonnes.

Last week the Navy said high-powered magnets and air balloons were among the possible options to lift the submarine. An undersea robot will also be used in the operation, it said.

The military has yet to offer an official explanation for the sinking of the decades-old submarine, which was delivered to the Southeast Asian nation in 1981.