Chinese Foreign Ministry spokesman Hong Lei told reporters, "The Chinese side expresses serious concern and firm opposition to the U.S. Senate's amendment to the National Defense Authorization Act which involves the Diaoyu Island (Senkaku Islands) and its affiliated islets," the official Xinhua News Agency reported.
The two Asian neighbors are locked over their respective claims to the Japanese-controlled islands in the East China Sea, also called Diaoyu Islands in China. Bilateral tensions have escalated since September when Japan, the main Asian ally of the United States, nationalized the islands in September.
The United States has so far not taken a position on the dispute, saying only that it should be settled peacefully.
However, Xinhua reported last Thursday's approval by the U.S. Senate of an amendment to the National Defense Authorization Act noted Japan has rights of administration over the islands and that "unilateral actions of a third party" would not affect its position.
Hong said the islands and its affiliated islets have always been the inherent territory of China since ancient times and that China has undisputed sovereignty over the Diaoyu Islands, Xinhua reported.
The report quoted the U.S. Senate measure as saying any armed attack "in the territories under the administration of Japan" would be met under the Treaty of Mutual Cooperation and Security.
Hong described the U.S.-Japan treaty a "product of the Cold War era" and that it should not go beyond bilateral scope, nor undermine the interests of a third party.
The spokesman, saying the United States has repeatedly stated that it will not take sides on territory disputes between China and Japan, added that Washington "should not send out signals that conflict with each other" but "do more things that are conducive to peace and stability in the region."
Japan Times said while the unanimously approved U.S. Senate measure is designed to counter attempts by China to challenge Japan's administration of the islands, it sidestepped as to who has ultimate sovereignty over the islands.
The amendment, offered by Sen. Jim Webb, D-Va., said the United States opposes any effort to coerce, threaten to use force or use force to resolve territorial issues, the report said. The amendment also reaffirmed U.S. commitment to the defense of territories under the administration of Japan.
"While the United States takes no position on the ultimate sovereignty of the Senkaku Islands, the United States acknowledges the administration of Japan over the Senkaku Islands," the amendment says. "The unilateral actions of a third party will not affect United States acknowledgement of the administration of Japan over the Senkaku Islands."
Senator Webb was quoted as saying that China over the years has taken increasingly aggressive actions to assert its claim over the islands as well as in a broad expanse of the South China Sea.
In an earlier "commentary," Xinhua said the U.S. Senate amendment "will boomerang" as China-Japan ties "plunge to their lowest in decades."
Xinhua said the amendment, which is yet to be approved by the U.S. House of Representatives and signed by President Barack Obama, could "embolden the Japanese rightists to continue defying the international order established after World War II."
The news agency said the amendment will also "impair the much-hyped U.S. pivot to Asia strategy, depriving Washington of the chance to gain advantages achieved by a peaceful and prosperous Asia."
The report said China's determination to defend its "territorial sovereignty should never be underestimated."