Sailors aboard the aircraft carrier USS George Washington man the rails as the ship pulls out of Hong Kong after a five-day port visit in this U.S. Navy handout photo dated November 14, 2011. (US Navy via Reuters)

A Chinese observation ship shadowed the US aircraft carrier John C. Stennis in the Western Pacific on Wednesday, the carrier's commander said, as it joined warships from Japan and India for drills close to waters Beijing considers its backyard.

The show of US naval power comes as Japan and the United States worry China is extending its influence into the Western Pacific with submarines and surface vessels as it pushes territorial claims in the neighbouring South China Sea, expanding and building on islands.

China has been angered by what it views as provocative US military patrols close to the islands. The United States says the patrols are to protect freedom of navigation.

Tokyo on Wednesday said a separate Chinese navy observation ship entered its territorial waters south of its southern Kyushu island. China said it was acting within the law and following the principle of freedom of navigation.

"There is a Chinese vessel about seven to ten miles away," Captain Gregory C. Huffman, commander of the Stennis, told reporters aboard the carrier after it recovered its F-18 jet fighters taking part in the exercise. The Chinese ship had followed the US vessel from the South China Sea, he added.

Chinese Foreign Ministry spokesman Lu Kang said he was unaware of the situation.

Beijing views access to the Pacific as vital both as a supply line to the rest of the world's oceans and for the projection of its naval power.

The 100,000-ton Stennis joined nine other naval ships including a Japanese helicopter carrier and Indian frigates in seas off the Okinawan island chain. Sub-hunting patrol planes launched from bases in Japan are also participating in the joint annual exercise dubbed Malabar.

The Stennis will sail apart from the other ships, acting as a "decoy" to draw it away from the eight-day naval exercise, a Japanese Maritime Self Defense Force officer said, declining to be identified because he was not authorized to talk to the media.

Blocking China's unfettered access to the Western Pacific are the 200 islands stretching from Japan's main islands through the East China Sea to within 60 miles of Taiwan. Japan is fortifying those islands with radar stations and anti-ship missile batteries.

By joining the drill, Japan is deepening alliances it hopes will help counter growing Chinese power. Tensions between Beijing and Tokyo recently jumped after a Chinese warship for the first time sailed within 24 miles of contested islands in the East China Sea.

The outcrops known as the Senkaku in Japan and the Diaoyu in China lie 137 miles northeast of Taiwan.

Wary of China's more assertive maritime role in the region, the US Navy's Third Fleet plans to send more ships to East Asia to work alongside the Japan-based Seventh Fleet, a US official said on Tuesday.

For India, the gathering is a chance to put on a show of force close to China's eastern seaboard and signal its displeasure at increased Chinese naval activity in the Indian Ocean. India sent its naval contingent of four ships on a tour through the South China Sea with stops in the Philippines and Vietnam on their way to the exercise.