Japan’s defense ministry announced late Friday that two Russian Navy ships had been sighted in waters near Taiwan and Japan’s Okinawa islands in the previous four days, following a similar announcement from Taiwan this week.

Taiwan’s defense ministry announced on Tuesday that it had sighted two Russian frigates off its eastern coast and had dispatched aircraft and ships to keep an eye on them.

Japan’s government said last month that repeated Russian military activity near Japanese territory, including joint drills with Chinese forces, posed “serious concern” for Japan’s national security.

Japan and Taiwan have joined the United States and its allies in imposing wide-ranging sanctions on Russia after its invasion of Ukraine last year.

The Japanese ministry said two Steregushchy-class frigates were first spotted 70 km (40 miles) southwest of Japan’s westernmost island of Yonaguni, in Okinawa prefecture neat Taiwan, on Tuesday morning.

The vessels sailed back and forth through the waters between Yonaguni and Taiwan, moved eastward and were last spotted on Friday in the waters between Miyako and Okinawa islands, it said, adding Japan dispatched two vessels to monitor the Russian ships.

Russia’s Interfax news agency reported on Tuesday that a detachment of ships of the Russian Pacific Fleet had entered the southern parts of the Philippine Sea to perform tasks as part of a long-range sea passage.