A Chinese observation ship shadowed the U.S. 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 U.S. 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.

Reuters reports that China has been angered by what it views as provocative U.S. 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 U.S. 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.