(Recasts with comments from U.S. official)
By Phil Stewart
QINGDAO, China, April 7 (Reuters) - U.S. Defense Secretary Chuck Hagel took a two-hour tour around China's sole aircraft carrier on Monday, in an unprecedented opening by Beijing to a potent symbol of its military power.
The carrier visit came at the start of Hagel's three day trip to China, his first as defense secretary, and was described by one U.S. official traveling with him as an apparently genuine gesture of transparency by the People's Liberation Army (PLA).
Hagel's tour of the Liaoning gave him a close-up look at the carrier's flight deck, medical facilities and living quarters and also allowed him to speak directly with Chinese sailors.
"We didn't see every space aboard the ship. But, yes, we felt this was an honest, genuine effort to be open about this brand new capability that they're trying to develop," said one U.S. official, speaking on condition of anonymity.
The official said Hagel was the first official visitor from outside China to be allowed on board the Liaoning, although that could not be immediately confirmed.
The 60,000-tonne Liaoning, a Soviet-era vessel bought from Ukraine in 1998 and re-fitted in a Chinese shipyard, is seen as a symbol of China's growing naval power and ambition for greater global influence.
The carrier has yet to become fully operational, however, and military experts say it could be decades before China catches up to the far superior and larger U.S. carriers - if ever.
The U.S. official said PLA officers were aware of the challenges ahead.
"On more than one occasion the officers that were with us said, quite frankly, that they know they have long way to go in naval aviation," the official said. "It's a difficult military capability to develop and to perfect."
Reporters traveling with Hagel, who landed in the port city of Qingdao after a trip to Japan, did not accompany him on the vessel - a rare visit to a sensitive Chinese military site.
"It's a sign of openness, of sincerity, that China has nothing to hide and wants to improve military relations with the United States," said Ni Lexiong, a naval expert at the Shanghai University of Political Science and Law.
"It will also be a good opportunity for the Americans to see the difference between the Liaoning and their aircraft carriers," Ni said, referring to the technological gulf between the two countries.
Chinese security experts said Beijing could be trying to quell U.S. criticism that it was not transparent about its military modernization, which Pentagon officials suspect is aimed at countering U.S. military capabilities.
Hagel's carrier visit was a surprise when it was publicly disclosed earlier on Monday. The United States requested the tour in January and China approved it in recent weeks, the U.S. official said.
CHINA GETS REGULAR ACCESS
Chinese military brass are no strangers to U.S. warships, including aircraft carriers.
Officers from the PLA are routinely flown to U.S. aircraft carriers en route to occasional port stops in Hong Kong, according to U.S. military officials.
Their U.S. counterparts provide tours of the ship and flight deck during operations - efforts U.S. diplomats say are geared to nudging China towards greater transparency about its capabilities.
Ian Storey, a Singapore-based regional security expert, said earlier that Hagel's visit would be "long on symbolism but short on actual operational capabilities".
"By showing him a vessel that was built in Ukraine in the 1980s and remains only a training platform that is still not fully operational, the Chinese will be keeping him away from their more sensitive capabilities, such as their missile programmes or submarine fleets," said Storey, of Singapore's Institute of South East Asian Studies.
China is building submarines, surface ships and anti-ship ballistic missiles, and has tested emerging technology aimed at destroying missiles in mid-air.
The carrier visit came a day after Hagel said he would use his first trip to China as defense secretary to press Beijing to use its "great power" wisely and respect its neighbors, who have been put on edge by the country's growing assertiveness in Asia's disputed waters.
China claims 90 percent of the 3.5 million sq km (1.35 million sq mile) South China Sea, which is believed to be rich in oil and gas. The Philippines, Vietnam, Malaysia, Brunei and Taiwan all claim parts of those waters.
China has a separate dispute with Japan in the East China Sea over uninhabited islets that are administered by Japan. China's decision in November to declare an air defence identification zone in the area that includes those islands sparked protests from the United States, Japan and South Korea.
China's official Xinhua news agency, in a commentary released as Hagel arrived, said Hagel had to show he was sincere about improving ties and the United States had to "assure its China friend of its intentions".
"As the world's second- and first-largest economies, China and America have a shared interest in a stable environment to facilitate economic prosperity. Neither of them, nor the global economy, can afford confrontation or conflict," Xinhua said.
Still, U.S. questions about China's military modernization go well beyond the carrier programme or even its naval capabilities. One issue Hagel is expected to raise on his trip to China is the need for greater dialogue on cyber warfare.
"We very recently shared with them some basic (U.S. military) doctrine with them on cyberspace ... they have not reciprocated with a like example," the official said. "We certainly hope that they will be able to do that."
RISKS OF A MISHAP
Hagel, in his talks in Japan over the weekend and last week at a gathering of Southeast Asian defense chiefs in Hawaii, had sought to reassure allies of the U.S. security commitment to the region and has promised frank discussions in China.
China, in turn, has repeatedly urged the United States not to take sides in any of its disputes in the region, and has watched warily as Washington moves to strengthen its military alliances, especially with Tokyo and Manila, as part of its strategic "pivot" toward Asia.
"Both sides are aware of the potential for military clashes, and the need to increase understanding and manage and reduce the risks, so this visit is a positive sign," said Zhu Feng, director of the International Security Programme at Peking University.
The risks of a mishap were highlighted in December when the American guided missile cruiser USS Cowpens had to take evasive action in the South China Sea to avoid hitting a Chinese warship operating in support of the Liaoning.
(Additional reporting by Ben Blanchard in Beijing and Greg Torode in Hong Kong; Editing by Dean Yates and Robert Birsel)