US Defence Secretary Chuck Hagel has toured China's first aircraft carrier, the Liaoning, at the beginning of a three-day visit to China.
Mr Hagel, who arrived in the port of Qingdao from Japan, is thought to be the first senior Western official to board the vessel.
China bought it from Ukraine in 1998 and has spent 10 years refitting it.
It is seen as a potent symbol of China's ambition to modernise its navy, amid a strategic shift in the region.
The fact that the US secretary of defence was allowed to step on board the carrier will be seen as a sign that the two countries may be willing to engage in more military co-operation, reports the BBC's Martin Patience in Beijing.
Washington has repeatedly called for more transparency from Beijing on its military spending, our correspondent adds.
US officials said that the defence chief's visit to the Liaoning at Yuchi naval base - which took place after a US request - lasted about two hours.
No further details were immediately available and journalists accompanying him on the China visit did not go with him.
The carrier was built in the 1980s for the Soviet navy but was never completed.
When the Soviet Union collapsed in 1991, the rusting hull - then called the Varyag - sat in dockyards in Ukraine.
A Chinese company with links to China's People's Liberation Army (PLA) then bought the Varyag, saying it wanted to turn the vessel into a floating casino in Macau.
In 2001 the ship was towed to China. The Chinese military confirmed in June 2011 that it was being refitted to serve as the nation's first aircraft carrier.
Earlier this year, it completed sea trials in the South China Sea, where China has overlapping territorial claims with several South East Asian nations.
Beijing's more assertive stance on this issue in recent years has led to a rise in tensions between China and its neighbours, particularly Vietnam and the Philippines.
China is also embroiled in a separate dispute over East China Sea islands that are controlled by Japan.
In Tokyo, Mr Hagel addressed regional territorial disputes, saying Chinese authorities should have "respect for their neighbours".
"You cannot... redefine boundaries and violate territorial integrity and sovereignty of nations by force, coercion and intimidation, whether it's in small islands in the Pacific or large nations in Europe," he said.
The Liaoning aircraft carrier has already attracted controversy. Late last year, Mr Hagel criticised China as "irresponsible" after the near-collision of a US warship and a Chinese naval vessel in the South China Sea.
The US said its guided missile cruiser, USS Cowpens, was operating in international waters on 5 December when the Chinese vessel - which was accompanying the Liaoning - forced it to take evasive action.
State-run newspaper Global Times, however, quoted an expert as saying that the US boat had been "harassing" the Liaoning as it carried out drills.