By Jonathan Marcus BBC defence correspondent

An accident on board a Chinese vessel on Friday appears to have involved a Ming class submarine - an obsolete boat used largely for patrol and coastal defence duties.

The Ming is essentially a modernised version of the Romeo class of boats - an old Soviet design, itself based on late-World War II German U-boats.

The first three Ming class boats entered service during the 1970s - but there were problems and one was scrapped after a fire.

China has not had much success with locally-built subs

Production resumed in 1987 but only after the boats were re-designed.

According to the Military Balance published by the International Institute for Strategic Studies in London, the Chinese Navy currently has some 16 Ming class boats in service.

The boat probably has a smaller crew than the 70 who are said to have died.

It is possible additional personnel were on-board for training purposes.

No success

The fact that the vessel did not sink suggests that the crew may have been poisoned by fumes, perhaps from a fire or from some mechanical malfunction in the boat's engines or life-support systems.

The Chinese Navy has not had much success with locally-built submarines.

Plans to develop a new series of diesel-powered boats - the Song class - appear to have been put on hold.

China already operates four comparatively modern Russian Kilo class boats and last May it agreed with Moscow to purchase at least eight more.