State Department spokesman Sean McCormack called the remarks attributed to General Zhu Chenghu "unfortunate" and said he hoped they did not reflect the views of the Chinese government.

"I haven't seen all the remarks but what I've seen of them, I'll say that they're irresponsible," McCormack told reporters.

The Financial Times and the Asian Wall Street Journal on Friday quoted Zhu, a professor at China's National Defence University, as issuing the threat at a briefing organised by a private Hong Kong organisation.

"If the Americans draw their missiles and position-guided ammunition on to the target zone on China's territory, I think we will have to respond with nuclear weapons," he reportedly said.

McCormack responded that the United States was no threat to China and the two countries had "probably the best US-China relationship we've seen in quite some time."

Secretary of State Condoleezza Rice has just visited Beijing and her deputy Robert Zoellick would be traveling there later this month to begin a strategic dialogue with the Chinese leadership.

"So we have, we believe, good, constructive relations with China," McCormack said.

"I think that, again, given the relationship of the US and China and the fact that the US is not a threat to China, that those remarks are certainly unfortunate."

A senior State Department official, who asked not to be named, said he was unaware of any contacts between the US embassy in Beijing and the Chinese government on the nuclear comments.

"But certainly the public expression of concern (by the United States) is important," the official said.

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