Japan controls the chain and calls them the Senkaku Islands, while China calls them the Diaoyu Islands.

According to Japanese broadcaster NHK, protests were lodged with the Chinese Embassy in Tokyo and in Beijing through the Japanese Embassy.

China's State Oceanic Administration, which oversees the country's Coast Guard, put out a statement late Monday saying the three ships "cruised within China's Diaoyu Islands territorial sea."

China's Defense Ministry, which is separate from the Oceanic Administration, posted a statement on its website confirming the Coast Guard ship movement.

Japan's Coast Guard said the Chinese vessels spent two hours in Japanese territorial waters.

It was the fourth time Chinese ships have entered Japan's waters this year, the Japanese Coast Guard said. There were 36 such incidents in 2016, it said.

"I made clear that our long-standing policy on the Senkaku Islands stands -- the US will continue to recognize Japanese administration of the islands and as such Article 5 of the US-Japan Security Treaty applies," Mattis said in a press conference with Japanese Defense Minister Tomomi Inada.

China responded quickly on Saturday to the US stance on the islands, saying it brings instability to the region.

"Diaoyu and its affiliated islands have been Chinese territory since ancient times. These are historical facts that cannot be changed. The so-called US-Japan security treaty was a product of the Cold War, and it should not harm China's territorial sovereignty and legitimate rights," Chinese Foreign Ministry spokesman Lu Kang said in a statement.

"We urge the US side to adopt a responsible attitude and stop making wrong remarks on the issue of the sovereignty of Diaoyu Islands," Lu said.

The US commitment to the Senkaus, which Mattis also reaffirmed in a Friday night meeting with Japanese Prime Minister Shinzo Abe, is not new. In 2014, the Obama administration also said the remote chain fell under the treaty.

In late 2013, Chinadeclared an air defense identification zone (ADIZ) over much of the East China Sea, including the Senkakus, despite objections from Tokyo and Washington.

Japan also has an ADIZ over the islands. Although the Senkakus are uninhabited, their ownership would allow for exclusive oil, mineral and fishing rights in the surrounding waters.

While the Senkakus remain a source of friction in the US-China relationship, Mattis' visit seemed to have soothed relations a bit in another area, the South China Sea.

Although the US defense chief said "China has shredded the trust of nations in the region" by fortifying disputed South China Sea islands, Mattis also said no increased US military maneuvers there were needed.

Lu, the Chinese Foreign Ministry spokesman, on Monday welcomed that stance.

"As for Mattis' comment that there is no need for large-scale military operations in the area and the issue should be resolved through diplomacy, these remarks deserve our affirmation," Lu said.

CNN's Yuli Yang, Steven Jiang and Yoko Wakatsuki contributed to this report.

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Chinese ships sail near disputed Japanese islands - CNN