TOKYO - A Chinese airplane was spotted Thursday above small islands controlled by Tokyo but claimed by Beijing, the first time a Chinese aircraft allegedly violated airspace over the islands and the latest in a brewing territorial spat.

Japan levied a formal protest later in the day, but China said it was merely carrying out a normal operation.

The chief government spokesman said the Chinese plane entered Japanese airspace in the morning. The Defense Agency said four Japanese F-15 jets headed to the area Thursday morning, but the Chinese plane, a Y-12, a nonmilitary aircraft, was nowhere to be seen.

The islands, known as Senkaku in Japanese and Diaoyu in Chinese, have been at the center of a territorial dispute. The purchase of the islands by the Japanese government this year from private Japanese owners set off massive anti-Japanese rallies in China.

"I want to stress that these activities are completely normal. The Diaoyu and its affiliated islands are China's inherent territory since ancient times," Chinese Foreign Ministry spokesman Hong Lei said. "China requires the Japanese side stop illegal activities in the waters and airspace."

It was the first time a Chinese plane entered Japanese airspace over the disputed islands, as the two previous violations were by a Soviet plane in 1979 and a plane from Taiwan in 1994, Defense Agency official Takashi Inoue said.

U.S. Assistant Secretary of State Kurt Campbell, while noting he did not yet know the specifics of the latest incident, urged calm, while noting the United States does not "take a position on the sovereignty of these islands."

"The security treaty between the United States and Japan applies to any provocative set of circumstances," he told reporters at the U.S. Embassy in Kuala Lumpur during an Asian tour. "We are encouraging all sides to take appropriate steps so that there will be no misunderstanding or miscalculation that could trigger an environment that would be antithetical to peace and stability."