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In a rare face-to-face encounter with an American defense secretary, a small group of Chinese officers held a spirited and sometimes pointed debate Thursday with Donald H. Rumsfeld over the two countries' clashing views about the size and meaning of China's military buildup.

Stressing a theme that he repeated throughout his three-day visit to Beijing, Rumsfeld told students and faculty at the Academy of Military Science that the United States and other countries are concerned not so much that China is expanding its military but that it has been vague about why.

"China of course is expanding its missile forces and enabling those forces to reach many areas of the world well beyond the Pacific region," Rumsfeld said in opening remarks before engaging in a give-and-take session with several officers, including the head of the military academy.

"Those advances in China's strategic strike capability raise questions, particularly when there's an imperfect understanding of such developments on the part of others," Rumsfeld added. A day earlier he visited the headquarters of China's strategic missile forces -- the first by a foreigner.

Chinese Foreign Ministry spokesman Kong Quan (search) said later, "We have said on many occasions that China's move to step up its defense capability is quite legitimate and reasonable."

Kong also said the two countries had "agreed to step up military exchanges and relations, especially on military institution education and visits of warships."

Later Thursday, Rumsfeld flew to Seoul, South Korea, where he was attending an annual meeting of U.S. and South Korean defense officials Friday to review their alliance and its recent turbulence.

The Pentagon has begun pulling thousands of U.S. troops out of South Korea, where it has maintained a contingent of about 37,000 troops for decades -- a legacy of the Cold War amid concern that communist North Korea might attempt to reunite the two Koreas by launching an all-out attack.

Gen. Leon J. LaPorte, commander of U.S. forces in Korea, told reporters Thursday evening that by the end of this year 8,000 of the 12,500 troops designated for withdrawal will have left South Korea.

A brigade of the 2nd Infantry Division, which is the main U.S. Army combat force in Korea, has already moved to Fort Carson (search), Colo., after serving a yearlong tour in Iraq. LaPorte said there is no current intention to reduce the American force beyond the 12,500 already designated to go.

In an interview with American reporters traveling with Rumsfeld, LaPorte said the South Korean government wants to review the command arrangement for the combined U.S.-South Korean military force that is under South Korean control during peacetime but would switch to U.S. control if war broke out.

"It is natural for a country, as it develops capabilities, to want to become more predominant in their own national security," LaPorte said. "So this is a natural evolution. We have supported the Republic of Korea for 50 years. They have the 12th largest economy in the world. So it's natural for them to say, `Listen, we appreciate the support we received; we are now capable of doing more things and taking a more predominant role."'

In Beijing, Rumsfeld made a point on several occasions of saying that China appears to be understating the size of its defense budget, and that this has created suspicions about its intentions.

During his give-and-take at the Academy of Military Science, a Chinese officer challenged him on this point, saying the defense budget had been rising in part to make up for insufficient defense investments in the past. The exchange was recounted for reporters later by Lawrence Di Rita, Rumsfeld's spokesman, who was present. Reporters were required to leave the room shortly after the question-and-answer session began.

The Chinese officer asked Rumsfeld why he was bothered by the budget increases, and Rumsfeld said he was not worried, although he went on to complain that China had been sending mixed signals about its interest in improving relations with the United States, according to Di Rita's account.

Rumsfeld accused China of teaming up with Russia to pressure Uzbekistan into telling U.S. forces to leave Uzbekistan last summer and of excluding the U.S. military from a search-and-rescue exercise in Hong Kong.

Another Chinese officer challenged Rumsfeld on the Uzbekistan (search) matter, saying the Pentagon was at fault for being insufficiently open about what its forces were doing in Uzbekistan and how long they intended to stay, according to Di Rita. Rumsfeld denied that U.S. purposes there were unclear.