A total of five Chinese warships, including destroyers and frigates, were spotted transiting waterways near Japan, a United States security treaty ally in East Asia, in the past week.

The Defense Ministry of Japan reported the Chinese naval movements near the country’s territorial waters, which are measured up to 12 nautical miles from its coasts. China has the largest navy in the world by hull count, with over 370 ships and submarines in service.

This came after Russia, a quasi-ally of China, sent a submarine and a rescue towboat to the waters between Yonaguni and Iriomote, part of Japan’s Southwest Islands, on Tuesday, which was the first time Japan has sighted a Russian submarine transiting in that area.

China and Russia have been conducting military activities near Japan recently, including joint strategic aerial patrols on November 29 to 30, which involved bombers capable of delivering nuclear weapons, as well as fighter jets, spy planes, and aerial refueling aircraft.

In its annual report, the Japanese Defense Ministry said China has intensified its activities in the East China Sea and the Pacific Ocean. While continuing its war in Ukraine, Russia has been engaging in joint military activities with China involving aircraft and vessels.

On Wednesday, the Chinese Type 052D destroyers CNS Xiamen and CNS Nanjing, as well as the Type 054A frigate CNS Huanggang, transited from the East China Sea to the Philippine Sea through the Miyako Strait, a major waterway in Japan’s Southwest Islands.

Another Type 054A frigate, CNS Yiyang, sailed the Miyako Strait northward on Sunday, following a southbound transit on November 28. From the Chinese military‘s perspective, this waterway is one of its major gateways for deployment to the wider Pacific Ocean.

The U.S. views the Miyako Strait as part of the first island chain, which extends from Japan southward to the Philippines via Taiwan. It is one of three layers that uses allied or friendly territories to constrain China’s military activity in the Western Pacific Ocean.

To the north in the Tsushima Strait, which lies between the Japanese archipelago and the Korean Peninsula, a Chinese Type 054A frigate, CNS Zaozhuang, transited the waterway northeastward and southwestward on November 28 and November 29, respectively.

This strait links the Sea of Japan, also known as the East Sea in South Korea, and the East China Sea. Vessels assigned to the Russian military also use the waterway for transit to and from their base in the country’s Far East region in the event of overseas deployment.

Newsweek has reached out to the defense ministry in Beijing for comment by email.