China's first aircraft carrier embarked on its inaugural sea trial Wednesday, Beijing said, prompting the United States to call for an explanation amid concerns about the nation's military expansion.

Beijing only recently confirmed it was revamping an old Soviet ship to be its first carrier and has sought to play down the vessel's capability, saying it will mainly be used for training and "research".

The voyage 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 defence ministry said the carrier's first sailing would be brief, and that the ship would afterwards return to the northeastern port of Dalian for more "refit and test work".

Andrei Chang, head of the Kanwa Information Centre, which monitors China's military, told AFP it would probably test whether the engines worked, and that on-off sea trials were likely to continue for another year or two.

"We would welcome any kind of explanation that China would like to give for needing this kind of equipment," US State Department spokeswoman Victoria Nuland told reporters when asked if the carrier would raise regional tensions.

"This is part of our larger concern that China is not as transparent as other countries. It's not as transparent as the United States about its military acquisitions, about its military budget," she said.

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.

Earlier this year, China announced military spending would rise 12.7 percent to 601.1 billion yuan ($91.7 billion) in 2011.

In January it revealed it was developing its first stealth fighter jet, and it is also working on an anti-ballistic missile capable of piercing the defences of even the most sturdy US naval ships.

The rapid expansion has caused concern among China's neighbours. Last week, Japan voiced concern over the country's widening naval reach and what it called the "opaqueness" of Beijing's military budget.

However, Beijing has repeatedly sought to alleviate fears over its pursuit of sophisticated weaponry and the official Xinhua news agency said the new carrier posed no threat to any other country.

"Building a strong navy that is commensurate with China's rising status is a necessary step and an inevitable choice for the country to safeguard its increasingly globalised national interests," it said.

"Even if China developed an aircraft carrier with full combat capacity in the future, it will not pose any threat to other countries."

China only provided the first official acknowledgment of the carrier in June when Chen Bingde, the nation's top military official, gave an interview to a Hong Kong newspaper.

But media reports and military analysts had said for years China was busy refitting an old Soviet aircraft carrier.

China has become increasingly assertive in its claims over the East China Sea and South China Sea, most of which it views as its maritime territory, but where several other Asian nations have competing claims.

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

China reportedly bought the 300-metre (990-foot) carrier's immense armoured hull -- with no engine, electrics or propeller -- from the Ukraine in 1998.

"The Varyag will carry an advanced Chinese... radar system, which will only be featured on the next US carrier that will not join the US fleet until 2015, said Rick Fisher, a Chinese military expert at the US International Assessment and Strategy Centre.

According to Chinese state television, the carrier will not be officially named until it carries out its first formal duty.

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