China's first aircraft carrier began its second sea trial on Tuesday after undergoing refurbishments and testing, the government said, as tensions over maritime territorial disputes in the region ran high.

The 300-metre (990-foot) ship, a refitted former Soviet carrier called the Varyag, underwent five days of trials in August that sparked international concern about China's widening naval reach.

"China's aircraft carrier platform, after successfully completing its first sea trial in August, returned to the shipyard as planned for further refitting and testing," the defence ministry said in a brief statement.

"The work has been carried out and it set sail again on November 29 to carry out relevant scientific and research experiments."

Beijing only confirmed this year that it was revamping the old Soviet ship and has repeatedly insisted that the carrier poses no threat to its neighbours and will be used mainly for training and research purposes.

But the August sea trials were met with concern from regional powers including Japan and the United States, which called on Beijing to explain why it needs an aircraft carrier.

Tuesday's announcement comes against a background of heightened tensions over maritime disputes in the Asia-Pacific region, where China's growing assertiveness has put it on collision course with the United States.

President Barack Obama this month irritated Beijing with a drive to enhance the US role as a regional power, positioning Marines in northern Australia and pushing for a potentially transformational trans-Pacific trade pact.

Beijing sees the initiatives as intruding into its own sphere of influence, with the dispute over the South China Sea putting the two major world powers' differences into stark focus.

China claims all of the strategic area, as does Taiwan, while four Southeast Asian countries declare ownership of parts of it, with Vietnam and the Philippines accusing Beijing's forces of increasing aggression there.

"China sticks to the path of peaceful development and upholds a defence policy that is defensive in nature. China will always be an important force to maintain regional peace," Chinese foreign ministry spokesman Hong Lei said Tuesday.

The region is a conduit for more than one-third of the world's seaborne trade and half its traffic in oil and gas, and major petroleum deposits are believed to lie below the seabed.

The announcement of the carrier's second sea trial comes after Beijing said last week it would conduct "routine" naval exercises in the Pacific Ocean before the end of November.

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

The PLA -- the world's largest active military -- 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.