The unarmed aircraft took off from Guam on Monday and the flight was previously scheduled as part of a routine exercise in the area, a Department of Defense spokesman said on Tuesday.

"Last night we conducted a training exercise that was long-planned. It involved two aircraft flying from Guam and returning to Guam," said Colonel Steven Warren, a spokesman for the department.

No flight plan was submitted beforehand to the Chinese and the mission went ahead "without incident", with the two aircraft spending "less than an hour" in the unilaterally-declared Air Defense Identification Zone (ADIZ), he said.

An anonymous official told the AFP news agency that the planes were B-52 bombers - the largest aircraft in the US military's arsenal.

China last week announced the expanded air defense zone amid a growing territorial dispute with Japan over an island chain in the East China Sea.

The area also includes waters claimed by Taiwan and South Korea, which also have both expressed their displeasure at Beijing's move.

Under the rules declared by China, aircraft are expected to provide a flight plan, clearly mark their nationality and maintain two-way radio communication to allow them to respond to identification inquiries from Chinese authorities.

Japan, the United States and several other governments have criticized China's announced air defense zone.

NTJ/BA