BEIJING (AFP) ― China on Monday branded a U.S.-Japan security treaty “a product of the Cold War” after Washington reaffirmed its commitment to Japan in its territorial dispute with China over the Senkaku Islands, known in Chinese as the Diaoyu.

The amendment, attached to the National Defense Authorization Bill, noted that while the United States “takes no position” on the ultimate sovereignty of the territory, it “acknowledges the administration of Japan over the Senkaku Islands.”

It added that “unilateral actions of a third party” would not affect its position.

The legislation passed last week reaffirmed the U.S. commitment to Japan under the Treaty of Mutual Cooperation and Security and warned that an armed attack against either party “in the territories under the administration of Japan” would be met in accordance with its provisions.

“The Chinese side expresses serious concern and firm opposition to the U.S. Senate’s amendment to the National Defense Authorization Act,” Foreign Ministry spokesman Hong Lei told reporters.

“The Diaoyu islands and affiliated islands have been China’s inherent territory since ancient times. China has indisputable sovereignty over them.”

Hong said the legislation violated Washington’s repeated pledge to not take sides in the dispute.

“The U.S.-Japan security treaty is a product of the Cold War and should not go beyond the bilateral scope or undermine the interests of a third party,” Hong said.

“We hope the U.S. side will bear in mind the broader interests of peace and stability in the region, honor its words with actions and refrain from sending self-contradictory, wrong signals.”

The sovereignty of the islands has been a source of friction for decades, but the row erupted earlier this year after the nationalist governor of Tokyo said he wanted to buy them for the city, forcing the Japanese government to nationalize them.

Chinese vessels have been spotted in and around the territorial waters every day for the last month.

Both sides have publicly refused to back down on their respective claims to the Japan-controlled islands.