TOKYO — U.S. military presence in the Pacific is essential to restrain Chinese assertiveness, Defense Secretary Robert Gates said Friday.
Gates also described China's technology advances as a challenge to U.S. forces in the region.
The defense secretary's comments are likely to add to tensions over political and economic quarrels between the two superpowers just days before Chinese President Hu Jintao visits the United States.
President Barack Obama hosts Hu for a state visit on Jan. 19. U.S. officials say Obama will raise geopolitical problems such as Iran and North Korea as well as trade issues that bedevil ties between the world's two biggest economies.
Setting the tone for friction during the summit over the huge trade imbalance in Beijing's favor, U.S. Commerce Secretary Gary Locke complained on Thursday that China often failed to keep promises to open its markets and called for a "more equitable commercial relationship."
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Gates, in Japan after a visit to China earlier this week, said in a speech that advances by China's military in cyber and anti-satellite warfare technology could challenge the ability of U.S. forces to operate in the Pacific.
While saying he did not see China as an "inevitable strategic adversary", Gates stressed the importance of U.S. military ties with Japan, where about 49,000 U.S. military personnel are stationed.
Without the forward presence of U.S. troops in Japan, China "might behave more assertively towards its neighbors", he said.
Gates cited a territorial dispute between Japan and China that flared last year, calling it an example of why the U.S. alliance with Japan was so important.
Differences such as those over markets would always weigh on ties, some Chinese analysts said.
"With irreconcilable interests, it is impossible to eliminate policy differences, which limits the good relations," Wu Xinbo, a researcher at the Center for American Studies at Fudan University in Shanghai wrote in the English-language Global Times on Friday.
"Today, China is disappointed, dissatisfied and confused by the series of hardline policies against China in the second year of the Obama administration. China is worried that this is a sign of a current or future major reversal in U.S. policy and strategy toward China," the researcher added.
Gates' warning came days after China held its first test flight of a stealth fighter jet while Gates was in Beijing on a trip aimed at easing strained military ties.
China also plans to develop aircraft carriers, anti-satellite missiles and other advanced systems which have alarmed the region and the United States, the dominant military power in the Pacific.
"Questions about (China's) intentions and opaque military modernization program have been a source of concern to its neighbors," Gates told university students in Tokyo.Story: U.S. to renew military ties with a rising China
Gates said that even as the military relationship between the U.S. and China improves, at least one area of disagreement continues: "freedom of navigation."
That's a euphemism for the U.S. view that it has the right to sail its ships in waters that China claims as restricted.
Freedom of shipping and commerce have been basic principles for the United States since its founding, Gates pointed out.
He also told students that China's military sometimes does things without telling the country's senior political leadership. The communist party has firm control over the military, but "sometimes there are disconnects," Gates said.
He mentioned this week's flight test of the new J-20 stealth fighter as one such example. Gates said Hu did not appear to know about the test until Gates asked him about it.
He said the incident was a worry, highlighting the importance of U.S.-China dialogue on military issues with both civilian and military officials.
While China's unveiling of the stealth fighter this week may have grabbed headlines, foreign powers are more worried about a growing naval build-up, especially as China has disputes over maritime boundaries with many of its neighbors.
But it will not only be military strains that set the tone of Hu's talks in Washington. He will face tough questions over China's economic policies.
Locke's critical remarks followed a speech by U.S. Treasury Secretary Timothy Geithner, who urged Beijing to move faster in allowing its currency to appreciate, to remove other trade barriers and to revise policies that forcefully tilt the China market playing field in the favor of Chinese firms.
China has said it would reform the yuan at its own pace, and on Friday reiterated it would not bow to foreign demand for faster gains in the currency.
"In undertaking reform of the exchange rate formation mechanism for the renminbi ... that is based on China's own developmental interests and needs, and is not in response to demands from another country," Cui Tiankai, a vice foreign minister, said on Friday.
"Of course, in doing this, that can benefit both China's own reform and opening up and development, and also trade and economic relations with other countries, including with the United States," he added, speaking at a forum hosted by the Foreign Ministry.
Many U.S. lawmakers direct their ire at China's currency policy. They contend China deliberately undervalues its yuan by as much as 15 percent to 40 percent to give its companies an unfair price advantage.
New trade figures released on Thursday showed the U.S. trade deficit with China alone totaled $252 billion during the first 11 months of 2010, keeping it on track to surpass the annual record of $268 billion in 2008.
The Associated Press and Reuters contributed to this report.