The rise in scrambles caused by China was in sharp contrast to an overall 11 percent decline in such aircraft moves to 141 times in the year to March 2005, the Defense Agency said.

"We cannot tell what was behind the gain but we can at least tell the gain was remarkable," an agency spokesman said.

Jiji Press said the sharp increase indicated a stepping up of reconnaissance and intelligence operations by Chinese aircraft near Japanese airspace.

"There is a possibility that it was related to the territory issue or China's natural gas exploration project," a senior official of the agency was quoted by Jiji as saying.

Despite Japan's strong protest, China began drilling for oil and gas in 2003 near the Japanese exclusive economic zone -- a delineation not recognized by China -- in the East China Sea.

The gas fields lie in the area of a bitter territorial dispute -- islands lying between Taiwan and Japan which are known as Senkaku in Japanese and Diaoyu in China.

Relations between Japan and China have slid to a three-decade low with major anti-Japanese protests in China accusing Tokyo of not atoning for its wartime atrocities after approving a nationalist textbook.