Three Chinese Coast Guard ships entered waters near a chain of Islands claimed by both China and Japan in the East China Sea on Monday, according to authorities from both sides.
Japan controls the chain and calls them the Senkaku Islands, while China calls them the Diaoyu Islands.
The sailing comes just days after U.S. Defense Secretary James Mattis reaffirmed America’s commitment to defending Japan and its disputed areas.
According to Japanese broadcaster NHK, protests were lodged with the Chinese Embassy in Tokyo and in Beijing through the Japanese Embassy.
Tensions have flashed numerous times in recent years over the disputed areas, including face-offs between Japanese and Chinese air and naval forces that have been termed dangerous by both sides.
This aerial shot taken on September 15, 2010, shows the disputed areas, known as Senkaku in Japan and Diaoyu in China, in the East China Sea.
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 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.
During a press conference Saturday in Tokyo, Mattis said the US would defend the islands with Japan.
“I made clear that our long-standing policy on the Senkaku Islands stands — the U.S. will continue to recognize Japanese administration of the islands and as such Article 5 of the U.S.-Japan Security Treaty applies,” Mattis said in a press conference with Japanese Defense Minister Tomomi Inada.
China responded quickly on Saturday to the U.S. stance on the islands, saying it brings instability to the region.
Read more: CNN