The Ming class submarines are based on ageing designs

Military analysts are puzzling over what caused the deaths of all 70 crew members on board a conventionally-powered Chinese submarine.

The accident - which occurred in Chinese territorial waters off its north-eastern coast - was the result of "mechanical problems" during a training exercise, China's official news agency said.

But the brief statement left many questions unanswered, and some experts are speculating that the deaths may have been the result of a gas leak.

It remains unclear why none of the crew were able to escape, why the boat was recovered so quickly, and exactly when and where the accident took place.

It is the worst submarine accident since August 2000, when the Russian nuclear-powered Kursk sank with its 118 crew in the Barents Sea.

The Chinese submarine did not sink during the accident and it has now been towed to an unnamed port.

Toxic gas theory

The vessel - known as No 361- was an indigenous Ming-class submarine, that normally has a crew of nine officers and 46 men, one military expert said.

The report by China's state-run media leaves many questions unanswered Rupert Wingfield-Hayes BBC Correspondent, Beijing China's 'obsolete sub'

Wendell Minnick, a Taipei-based analyst, was quoted by Reuters saying it was "very strange" that the vessel appeared to be carrying an extra 15 people.

"A fire or a collision is a possibility. If it was a torpedo that blew up, it probably would have sunk it," he added.

Michael McGinty, a defence analyst at the Royal United Services Insitute in London, said it was unusual for accidents on this scale to happen near the surface.

One possible scenario might be that the submarine's batteries may have leaked acid which mixed with seawater, producing toxic chlorine that could have killed the crew, he told the BBC.

But Mr McGinty described the deaths as "very mysterious"

It is one of the worst naval accidents in Communist China's history, according to military sources.

Ming-class 'obsolete'

The Chinese Central Military Commission has sent its condolences to families of officers and crew.

Given the Chinese military's notorious secretiveness, it's perhaps not surprising that the information is vague, the BBC's Rupert Wingfield-Hayes in Beijing says.

The vessel is believed to be built from Soviet designs based on German submarines dating back to World War ll.

While the Ming-class boats are entirely obsolete by modern standards, they are a relatively inexpensive option for patrol and coastal defence duties, and they are still being built.

The Chinese were reportedly negotiating with Russia last year to buy eight 636 Kilo-class vessels, equipped with anti-ship missile systems, in a $1.6 bn deal.

According to reports, China's own submarine manufacturing programme is in difficulty, particularly its efforts to develop the Song class guided-missile submarine.

Jane's Defence Weekly says the first Song started sea trials in 1995, but proved a failure.

China's navy has also reportedly experienced operating problems because of inadequate crew training.

Intelligence reports say Beijing increased military spending last year to $20 bn.