US Defense Secretary Donald Rumsfeld has again accused China of understating its defence spending, hours before he arrived in Beijing for talks.
It is Mr Rumsfeld's first trip to China since taking office
Mr Rumsfeld said China's understating of its defence build-up - which Beijing denies - was fuelling suspicions about its motives.
China's official military spending this year is $30bn, but the Pentagon said in June that the real figure was $90bn.
Mr Rumsfeld is in China to hold talks with President Hu Jintao, on Wednesday.
Mr Rumsfeld's trip comes ahead of a planned visit to China next month by US President George W Bush.
"China is an important country in the region; it's a country that's increasingly important in the world," Mr Rumsfeld told reporters on Tuesday.
But he added that its development had created "somewhat of a tension" for China's leaders as they coped with new influences and ideas.
Mr Rumsfeld said Beijing would have to make choices between its desire for economic growth and efforts to control foreign influences and access to information.
"Obviously, those of us in the United States and in other countries around the world hope that they make choices towards a more open society," he told reporters.
China has consistently increased its defence spending since the 1990s, but Chinese officials say the increase is needed to modernise its armed forces and pay better salaries.
China also says its budget is dwarfed by US military spending, which last year totalled $440bn.
The US says China's emergence as a major power could alter the military balance in the region.
The US is particularly concerned about a build-up of Chinese missiles pointed at Taiwan, which it has threatened to seize by force if the island moves towards formal independence.
Mr Rumsfeld drew attention to China's arms spending in Singapore in June, and shortly afterwards a Pentagon report estimated that China's military spending was much greater than disclosed.
"I think it's interesting that other countries wonder why they China would be increasing their defence effort at the pace they are and yet not acknowledging it," Mr Rumsfeld said on Tuesday.
"It is almost as interesting as the fact that it is increasing at the pace it is," he said.
Mr Rumsfeld is expected to spell out his views at a speech on Wednesday at a training facility for the Communist Party leadership.
He will also address a military school on Thursday, and hold discussions with his Chinese counterpart, Cao Gangchuan.
It is not just Mr Rumsfeld that has a lot to talk about, according to the BBC correspondent in Beijing, Daniel Griffiths.
China is wary of the considerable US presence in Asia, and ties have also been strained since the collision between an American surveillance plane and a Chinese fighter jet in 2001.
Mr Rumsfeld said he probably would have visited China sooner had it not been for this - this is his first visit since he took office in 2001.