The Chinese navy's first aircraft carrier has begun its sea trials, the state-run Xinhua news agency has said.
It quoted military sources as saying that the refitted former Soviet warship left its shipyard in the north-east and the trial "would not take a long time".
The move is likely to raise fresh concerns over China's rapid military build-up.
Beijing is currently involved in several maritime territorial disputes, particularly in the South China Sea.
The aircraft carrier left its shipyard at Dalian Port in northeast Liaoning Province on Wednesday morning, Xinhua reported.
"Military sources said that the first sea trial was in line with the schedule of the carrier refitting project," it said.
"After returning from the sea trial, the aircraft carrier will continue refit and test work."
Xinhua did not provide any further details.
The BBC's Michael Bristow in Beijing says China is years away from being able to deploy this carrier as a potent military tool. Even so, the country's neighbours will be worried.
Many are involved in disputes with China over maritime borders - and they will be looking anxiously at Beijing's naval build-up, our correspondent says.
Run-ins at sea
The carrier in question is a former Soviet warship, which was formerly called the Varyag.
It is a relatively old design and it was not built by China. It was constructed in the 1980s for the navy of the USSR, but was never completed. When the Soviet Union collapsed in 1991, the rusting hull of the Varyag sat in dockyards in Ukraine.
As other Soviet warships were cut up for scrap, a Chinese company with links to China's People's Liberation Army (PLA) bought the Varyag claiming it wanted to turn it into a floating casino in Macau. It took several years to finally tow it all the way to China, where it was then taken to Dalian.
In June, the PLA confirmed that China's first aircraft carrier was under construction.
However, Beijing has recently sought to downplay the capabilities of the carrier, saying it would be used for training and research.
China's military is generally believed to be 20 years behind America's in its development. But in its rapid expansion, China is focusing on weapons designed to blunt US military power, analysts say.
The PLA has invested heavily in submarines. It is believed to be close to deploying the world's first "carrier-killer" ballistic missile, designed to sink aircraft carriers while they are manoeuvring at sea up to 1,500km (930 miles) offshore, and it is building its own stealth fighter aircraft along with advanced carrier-based aircraft built from Russian designs.
All of these can target US bases, US ships and US carriers in Asia. They will make it much more dangerous for US carrier fleets to operate close to China's coast, pushing them out further offshore.
Taiwan, Korea and Japan that look to the US for their security may start to question how much America can really protect them in future, analysts say.
A Japanese defence study last week expressed concern about what it called China's failure to explain its military ambitions.
In the past year, China has had maritime run-ins with Japan, Vietnam and also the Philippines over disputed territories in the area.