China and the United States fought a verbal skirmish in recent days over the possible use of nuclear weapons against each other, underscoring the often precarious relations between Beijing and Washington.

China fired the first salvo, a belligerent statement on July 15 by Maj. Gen. Zhu Chenghu to foreign correspondents. Zhu said China would aim nuclear weapons at American cities if U.S. forces intervened in a Chinese assault to prevent Taiwan from turning its de-facto separation from China into formal independence.

The American response on Tuesday was subtle but unmistakable, at the very end of a Pentagon report on China's military power. It warned that China should avoid a conflict over Taiwan involving the United States because that "would give rise to a long-term hostile relationship between the two nations — a result that would not be in China's interests."

In the Beijing briefing arranged by the Foreign Ministry, Zhu said: "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."

("Position-guided ammunition" looks like a bad translation. and probably meant "precision-guided munitions, or "smart bombs.")

"If the Americans are determined to interfere, then we will be determined to respond," Zhu said. "We Chinese will prepare ourselves for the destruction of all the cities east of Xian," an ancient capital in central China. "Of course," he asserted, "the Americans will have to be prepared that hundreds of cities will be destroyed by the Chinese."

The general, dean of Beijing's National Defense University, contended this was his personal view. The Foreign Ministry reinforced that just after the general spoke, suggesting a scripted ploy. No serving officer in China makes policy statements without clearance from the top levels of government.

That led to speculation about what the Chinese were up to. Gen. Zhu, aware that the Pentagon was about to issue a report critical of China's military buildup, may have mounted a pre-emptive strike. As he acknowledged, China lacks the forces to take on the United States with conventional weapons and thus might resort to nuclear arms.

Clearly, however, this was not a new threat. Ten years ago, Lt. Gen. Xiong Guangkai, then a senior officer on China's general staff, issued a similar warning. In the meantime, many Chinese have asserted that Washington would not put an American city at nuclear risk in a conflict over Taiwan and thus would not fight to defend the island.

A former commander of the U.S. Pacific Command, Adm. Dennis Blair, told the Washington Post: "They think it's good to have a mad dog in your closet who might scare your potential adversaries." Blair and other senior U.S. officers have personally but privately cautioned Chinese leaders in recent years not to miscalculate American capabilities and intentions.

Whatever Gen. Zhu's motives, the U.S. government took his threat seriously.

A State Department spokesman called his remarks "highly irresponsible."

The Pentagon's report on Chinese military power, which Congress requires annually, was in preparation long before Zhu issued the nuclear warning. Nonetheless, it noted that China has deployed or is in final development of ballistic missiles that could hit anywhere in the United States and addressed the issues raised by the general, in the context of China's threat to Taiwan.

The report contended that China "does not yet possess the military capability to accomplish with confidence its political objectives on the island, particularly when confronted with outside intervention," meaning by the United States.

Further, a war "could severely retard economic development," the report said, adding that "international sanctions against Beijing, either by individual states or by groups of states, could severely damage Beijing's economic development."

China has claimed spectacular economic growth rates of 7 to 10 percent in recent years.

Politically, a war over Taiwan could "lead to instability on the mainland."

The report noted that a record 58,000 domestic protests, many of them violent, erupted in China last year. A failure in an attack on Taiwan, the report said, "would almost certainly result in severe repercussions" for leaders who had advocated military action.

"If Beijing chose to use force against Taiwan prior to the 2008 Olympics," the Pentagon continued, "China would almost certainly face a boycott or loss of the games." The Olympics are scheduled to be held in Beijing, which Chinese leaders sought to boost their prestige at home and abroad.

The Pentagon's final word was to caution again: "Beijing must calculate the probability of U.S. intervention in any conflict in the Taiwan Strait."

Richard Halloran is a Honolulu-based journalist and former New York Times correspondent in Asia.