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, U.S. Defense Secretary Robert Gates said in prepared remarks on Friday during a visit to Japan.

Just days after meetings in China meant to bolster ties with its military, Gates expressed renewed concerns about a buildup by the People's Liberation Army, which flexed its muscle this week with its first-ever test flight of a stealth fighter jet.

Gates called China's military modernization program opaque and stressed that, without the U.S. military presence in Japan, China might behave more assertively toward its neighbors.

(Reporting by Phil Stewart; Editing by Edmund Klamann)

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