The US Air Force considered a plan to drop nuclear bombs on China during a confrontation over Taiwan in 1958, but was overruled by then-president Dwight Eisenhower, declassified documents showed Wednesday.

Eisenhower instead ordered military officers to initially use conventional bombs against Chinese forces if the crisis escalated, according to previously secret US Air Force history.

The president's instructions seemingly astounded the top Air Force brass, but the author of a declassified report said US policymakers recognized that atomic strikes had "inherent disadvantages" because of the danger of radioactive fall-out in the region.

The report on the crisis by Bernard Nalty, an Air Force historian at the time, included details of an initial plan to drop 10 to 15 kiloton nuclear bombs on Chinese airfields in Amoy (now called Xiamen) in the event of a Chinese blockade against Taiwan's so-called Offshore Islands.

"This was in accordance with the drift of Air Force thinking which considered nuclear weapons as usable as 'iron bombs,'" according to the report, written in June 1968 and released Wednesday by the National Security Archive.

The Archive, a non-governmental research institute located at the George Washington University in the US capital, collects and publishes declassified documents obtained through the US Freedom of Information Act.

"Of course, if there was a real war who knows what would have happened but there wasn't fortunately," archive senior analyst William Burr told AFP.

During Eisenhower's presidency there were crises on the status of the Offshore Islands in 1954 and 1958 but they did not lead to any serious military confrontation, Burr said.

Eisenhower "ordered the Air Force and Navy to prepare for conventional strikes as a show of determination," Nalty wrote, but "if the conflict escalated, nuclear strikes could have followed."

What led the White House to change the ground rules was the recognition that atomic strikes had "inherent disadvantages" -- fallout would cause civilian casualties both in China and on Taiwanese territory, Nalty wrote.

An important lesson from that crisis was that "armed forces must expect civil authority to impose tight controls on them in times of emergency," the report said.

Taiwan and China split in 1949 at the end of a civil war, but Beijing still sees the island as part of its territory awaiting reunification, by force if necessary.

The United States, obliged by law to offer Taiwan a means of self-defense if its security is threatened, is the leading arms supplier to the island despite switching diplomatic recognition from Taipei to Beijing in 1979.

CIA chief Michael Hayden said Wednesday that China's rapid military buildup was "troubling" as "it reinforces long-held concerns about Chinese intentions towards Taiwan."

"But even without that issue, we assess the buildup would continue -- albeit one that might look somewhat different," he said in a speech at Kansas State University.

A Pentagon report said this year that China had boosted total military spending in 2007 to more than twice its declared budget but Beijing dismissed it as an exaggeration, made in order to justify US sales of military hardware to Taiwan.

China-Taiwan political ties deteriorated during the past eight years under the rule of President Chen Shui-bian, who had irked Washington and Beijing with his pro-independence stance.

But Beijing-friendly Ma Ying-jeou, who took over from Chen last month, has vowed to improve relations with China, increase trade, tourism and transport links, and work on a peace treaty to end hostilities.