China's first aircraft carrier has begun its inaugural sea trial, the defence ministry said Wednesday, a move likely to stoke concerns about the nation's rapid military expansion.

Beijing only recently confirmed it was revamping an old Soviet ship to be its first carrier, adding to worries among its neighbours over the country's military build-up and growing assertiveness on territorial issues.

"According to trial plans, the first sea trial will not take long," the defence ministry said in a statement.

"After returning to the shipyard (in the port city of Dalian), the carrier will continue its refit and test work."

Beijing last month sought to play down the capability of its first carrier, saying the vessel would be used for training and "research".

Chen Bingde, the nation's top military official, only provided the first official acknowledgment of the aircraft carrier in a Hong Kong newspaper interview in early June.

But media reports and military analysts have said for years that the 300-metre (990-foot) ship was in development.

The ship, once called the Varyag, was originally built for the Soviet navy. Construction was interrupted by the collapse of the Soviet Union in 1991.

Andrei Chang, head of the Kanwa Information Centre, which monitors China's military, told AFP the trial would likely only last about an hour.

"The first sea trial is just for testing different items, like whether the engines work or not," he said, adding that from Soviet and French aircraft carrier experience, on-off sea trials would continue for another year or two.

Last week Japan voiced concern over China's growing assertiveness and widening naval reach and over what it called the "opaqueness" of Beijing's military budget.

The carrier project also comes amid heightened tensions over a number of maritime territorial disputes involving China, notably in the South China Sea, which is believed to be rich in oil and gas and is claimed by several countries.

The issue has heated up recently with run-ins between China and fellow claimants Vietnam and the Philippines, sparking concern among its neighbouring countries and the United States.

In September, a row erupted between Japan and China over the disputed Senkaku Islands, known as the Diaoyu Islands in Chinese, located in the East China Sea.

China reportedly bought the ship's immense armoured hull -- with no engine, electrics or propeller -- in 1998.

Chang pointed out that he had not seen any form of arresting gear on the carrier -- the mechanical system that rapidly decelerates a plane as it lands -- in photos he had seen so far.

"That's very strange. It's very complicated technology and the Chinese haven't installed it -- we couldn't find it," he said.

"That means they can't land any fighters, probably only helicopters."

China's People's Liberation Army -- the largest armed force in the world -- is extremely secretive about its defence programmes, which benefit from a huge and expanding military budget boosted by the nation's runaway economic growth.

The PLA also operates the country's navy.

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