Chinese Foreign Ministry spokesman Lu Kang on Saturday called on the US to avoid discussion of the issue and reasserted China's claim of sovereignty over the tiny uninhabited islands, known in Japanese as the Senkaku and Chinese as Diaoyu, AP reported.
The 1960 US-Japan treaty is "a product of the Cold War, which should not impair China's territorial sovereignty and legitimate rights," Lu said.
"We urge the US side to take a responsible attitude, stop making wrong remarks on the issue involving the Diaoyu islands' sovereignty, and avoid making the issue more complicated and bringing instability to the regional situation," Lu added.
On his first trip to Asia as secretary of defense, Mattis explicitly stated in Tokyo that Washington will stick to its previous stance that the US-Japan security treaty applies to defending Japan's continued administration of the Senkaku islands.
The islands that lie between Taiwan and Okinawa were under US administration from the end of World War II until their return to Japan in 1972. China cites historical records for its claim, and Japan's move to nationalize several of the islands in 2012 set off anti-Japanese protests in China and prompted the government to dispatch ships and planes to the area around them as a challenge to Japanese control.
China also registered its displeasure with Mattis' remarks Friday in South Korea that the US is committed to carrying through on a deal to deploy a high-end US missile defense system to South Korea this year.
Beijing objects to the system because its powerful radar would allow it to peer deep into northeastern China, possibly allowing it to observe Chinese military movements.
At a Friday news conference, Lu said China's "resolute opposition to the deployment ... remains unchanged and will not change."
The deployment "will jeopardize security and the strategic interests of regional countries, including China, and undermine the strategic balance in the region," Lu said.