A total of 24 Russian warships have been operating in the Sea of Japan and the southern part of the Sea of Okhotsk since Feb. 1, the Defense Ministry said Tuesday, in what defense chief Nobuo Kishi called part of “unusual” large-scale military exercises.

"The move comes in concert with Russian forces' recent activities in areas near Ukraine," Kishi told a news conference Tuesday, noting that Russia's military was believed to be stepping up its activities to highlight that it remains capable of operating both on the east and west of the country at the same time.

Russia has more than 100,000 troops massed near Ukraine, stoking fears of an impending invasion.

"It’s unusual for Russia to carry out large-scale military drills at this time of year," Kishi said. "We are closely watching the activities, including those related to the Ukraine situation, with grave concern."

The defense minister said the activity close to the Japanese archipelago had been detected by Maritime Self-Defense Force destroyers and a P-3C anti-submarine and surveillance plane. The 24 Russian vessels included destroyers, frigates and submarines, the ministry announced.

The Russian Defense Ministry announced Feb. 1 that vessels from its Pacific Fleet had left port to conduct air-defense training, joint maneuvering and firing exercises in the Sea of Japan and Sea of Okhotsk.

Russia has repeatedly denied any plans to invade Ukraine, calling the claims “hysteria” stirred up by the United States, Japan’s top ally. Moscow announced Tuesday that some of its military units were returning to their bases after exercises near Ukraine, but it was not clear how many were being withdrawn and where they would go.

Later Tuesday, Prime Minister Fumio Kishida said he had spoken with Ukrainian President Volodymyr Zelenskiyy over the telephone, with the two leaders agreeing to pursue diplomatic efforts to help ease the soaring tensions.

Speaking to reporters, Kishida also said Japan planned to take “appropriate” steps — including potential sanctions — if Russia were to invade Ukraine.