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 on Friday during a visit to Japan.

Just days after meetings in China meant to bolster ties with its military, Gates 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.

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 said, addressing university students in Tokyo.

The comments were part of a broad speech drawing attention to the importance of U.S. military ties with Japan, where roughly 49,000 U.S. military personnel are stationed. Without the forward presence of U.S. troops in Japan, China might behave more assertively toward its neighbors, he said.

Gates cited a territorial dispute between Japan and China that flared up last year, calling it an example of why the U.S. alliance with Japan was so important.

Questions about China's growing role in the region manifest themselves in territorial disputes, most recently in the incident in September near the Senkaku Islands, Gates said, using the Japanese name for them.

In China they are called the Diaoyu islands.

Still, Gates said he did not see China as an inevitable strategic adversary.

President Hu Jintao will visit the United States next week, and Gates noted that Hu and U.S. President Barack Obama had called on their militaries to strengthen ties -- one of the most brittle links in overall Sino-U.S. relations.

It is precisely because we have questions about China's military -- just as they might have similar questions about the United States -- that I believe a healthy dialogue is needed, he said.

CYBER WARFARE?

Worry about China has convinced Japan of the need to repair an alliance frayed last year by a feud over a U.S. air base as well as efforts by Japan's Democratic Party-led government to forge a diplomatic stance less dependent on Washington.

In a sweeping update of its defense policies, Japan last month pledged tighter security ties with the United States.

Analysts say China's military advances appear designed to counter U.S. capabilities in the Pacific, despite assurances from Beijing that its modernization is peaceful.

Gates cited U.S. concerns about its cyber and space capabilities, specifically.

Advances by the Chinese military in cyber and anti-satellite warfare poses a potential challenge to the ability of our forces to operate and communicate in this part of the Pacific, Gates said.

Fortunately, (the) United States and Japan maintain a qualitative edge in satellite and computer technology.

Beijing carried out a watershed anti-satellite test in January 2007, using a ground-based missile to knock out one of its inactive weather satellites in high polar orbit. No advance notice of the test was given.

U.S. officials have also voiced concern about its investment in anti-ship missiles, which could challenge U.S. aircraft carriers in the Pacific.

Gates called for greater cooperation with Japan on missile defense, citing the North Korean threat.

North Korea's ballistic missiles -- along with the proliferation of these weapons to other countries -- require a more effective alliance missile defense capability, he said, ahead of a trip to Seoul later on Friday.

He said the U.S. troops in Japan also were an important deterrent for North Korea following the sinking of a South Korean warship and the shelling of a South Korean island last year.