"We think that everybody that conducts them or observes them or whatever should be very careful not to allow any incidents, not to allow any steps that might be provocative or might lead to an increase in tension," State Department spokesman Richard Boucher told reporters.

Taiwan air force jets staged rare landings on closed off freeways Wednesday as part of a major exercise to demonstrate the island's readiness in the event of an attack by China.

The Taiwanese manoeuvres came after China kicked off the largest war games of the year, simulating a fight for the control of air space over Taiwan.

China had said it would use the joint sea, land and air drills to demonstrate its ability to dominate air space over Taiwan, an essential element in any invasion.

Boucher said Wednesday he felt the military exercises were "a normal part of things" and added that he did not know whether Washington had asked Beijing and Taipei to avoid any provocations over the war games.

Taiwan said its exercise, part of the island's biggest annual military drill, is to "review the air force's capability in using freeways for emergency landings and logistic support in case of war."

Since pro-independence President Chen Shui-bian was re-elected in March, Beijing has stressed its long-standing vow to take Taiwan by force should the island try to formalise its split from China at the end of civil war in 1949.

Officials from the US mission in Taipei, known as the American Institute in Taiwan, reportedly witnessed the aircraft landings, designed to simulate a scenario in which the country's major airstrips would be destroyed by attack.

The United States has remained the leading arms supplier to Taiwan despite switching of diplomatic recognition from Taipei to Beijing in 1979.

Under a 25-year-old US law called the Taiwan Relations Act, the United States acknowledges Beijing's position that Taiwan is part of China but is bound by law to provide weapons to help Taiwan defend itself if its security is threatened.

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