The US sharply increased its criticism of China at the weekend, questioning the motives behind a Chinese military build-up, calling on the country's Communist rulers to embrace "a more open and representative government" and urging Beijing to do more to persuade North Korea to abandon nuclear weapons.

Donald Rumsfeld, US defence secretary, said the Pentagon's 2005 report on the Chinese military, to be published soon, concludes that China's defence spending is much higher than officially admitted, with its military budget ranking highest in Asia and third in the world.

Speaking at the annual Asian security conference of the London-based International Institute for Strategic Studies, Mr Rumsfeld criticised China's "significant roll-out" of ballistic missiles aimed at Taiwan.

He said China, although threatened by no other nation, also appeared to be expanding its long-range missile forces, "allowing them to reach targets in many areas of the world, not just the Pacific region".

"Though China's economic growth has kept pace with its military spending, it is to be noted that a growth in political freedom has not yet followed suit," Mr Rumsfeld said, after lauding the spread of democracy to Afghanistan, Iraq, Ukraine, Kyrgyzstan and Lebanon.

In Singapore yesterday, a senior US defence official said the US expected to decide within weeks whether to bring the issue of North Korea's nuclear weapons plans before the United Nations Security Council.

North Korea has refused for nearly a year to attend the six-nation talks designed to resolve the problem, and the US has become increasingly frustrated with the failure of China or South Korea to bring pressure to bear on Pyongyang.

Japan, the main US ally in the region, also appears to be taking a harder line on North Korea. "The opinion (in Japan) is gaining ground that we should do something like economic sanctions or bring this matter on the table at the UN," Yoshinori Ohno, Japanese minister of state for defence, told the conference.

China responded immediately to Mr Rumsfeld's comments on the Chinese armed forces. Cui Tiankai, a senior foreign ministry official, bluntly asked Mr Rumsfeld whether he truly believed China faced no threats and whether the US felt threatened by China's emergence.

Mr Rumsfeld repeated his view that China was not under threat, and insisted that the US did not feel threatened by the rise of China. But he added that the Chinese "will have to find ways to open up their political system" in a manner compatible with the open economic system needed for further growth.

Political and security analysts interpreted Mr Rumsfeld's words as a warning to China not to provoke a conflict in the Taiwan Strait and to put more pressure on North Korea to return to negotiations.

But some Asian commentators criticised what they saw as a simplistic American recipe for democratisation and said there were suspicions in Asia that the US wanted to destabilise China.

"My feeling is that the Chinese are alarmed," said Kishore Mahbubani, dean of the Lee Kuan Yew School of Public Policy in Singapore and a former ambassador to the UN. "For them the nightmare scenario is for the US to start believing that this tide of democracy should reach China."