He raised the issue Thursday in a speech to the Academy of Military Sciences in Beijing on the final day of a visit that has highlighted US concerns about a rapid mainland military buildup.

"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.

"Those advances in strategic strike capability raise questions, particularly when there is an imperfect understanding about such developments.

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"As a result, countries with interests in the region are asking questions about China's intentions."

Beijing, meanwhile, has asked Washington to help contain Taiwan's pro-independence forces.

Foreign Ministry spokesman Kong Quan said President Hu Jintao and military chief Cao Gangchuan both told Rumsfeld the United States has a responsibility to abide by the three joint communiques on Taiwan and respect Beijing's sovereignty over the island.

The one-China policy was stipulated in the three Sino-US joint communiques that laid the groundwork for normalization of diplomatic ties in the 1970s.

"Cao specifically pointed out that the Taiwan issue is connected to China's sovereignty and territorial integrity and is in China's core interest," Kong said. "We hope the US will implement its promise and, together with China, oppose and contain the Taiwan pro-independence force."

Kong called Rumsfeld's visit "very successful." He said officials from the two governments promised to take steps to "enhance mutual trust."

"The two sides agreed to step up military exchanges and relations, especially on military institution education and visits of warships," Kong said.

Rumsfeld said a recent Pentagon report found that China's military expenditure was two to three times greater than publicly acknowledged.

"Neighbors understandably wonder what the reason might be for the disparity between what they believe the reality to be and [China's] public statement," he said.

The Pentagon report in July said China is replacing older long-range ballistic missile systems with newer, more survivable missiles.

It said that over the next several years China will bring into service a new road-mobile intercontinental missile, an extended-range intercontinental missile and a new submarine-launched ballistic missile.

Rumsfeld's comments on China's nuclear strike capability came a day after he made an unprecedented visit to the headquarters of the Strategic Rocket Forces in Beijing.

US concerns about China's nuclear forces were piqued in July when National Defense University dean Zhu Chenghu said that if the United States targeted the mainland in a crisis over Taiwan, "I think we will have to respond with nuclear weapons."

But during Wednesday's visit to the strategic nuclear forces headquarters, its commander, Jing Zhiyan, assured Rumsfeld China is strictly adhering to a policy of "no first use" of nuclear weapons, a US defense official said.

Cao has said the need to raise the living standards of the country's poor makes it "impossible to massively increase" military spending. He put spending this year about US$30 billion (HK$234 billion), but noted the space program and other equipment spending was outside the defense budget.

AGENCE FRANCE-PRESSE, ASSOCIATED PRESS