TOKYO (AP) — A Chinese aircraft carrier entered Japan’s contiguous waters for the first time on Wednesday, leading Tokyo to convey its “serious concerns” to Beijing over China’s increasingly assertive military actions around Japan, officials said.

The Chinese carrier Liaoning, accompanied by two destroyers, sailed between Japan’s westernmost island of Yonaguni and nearby Iriomote, entering the country's co-called “contiguous zone,” the Defense Ministry said. That's an area of sea beyond a country's territorial waters in which it can still exercise some control over maritime traffic, reaching up to 24 nautical miles offshore.

The Liaoning's transit was part of a fleet movement Tuesday and Wednesday, during which Chinese warships also passed off the western coast of the disputed Japanese-controlled islets it calls the Senkakus, the ministry said. China claims the same islets, calling them the Diaoyus.

Japan’s Deputy Chief Cabinet Secretary Hiroshi Moriya told reporters that “the latest incident is absolutely unacceptable from the perspective of the national and regional security.”

Last month, Japan said that a Chinese Y-9 reconnaissance airplane violated Japanese airspace. Days later, a Chinese survey ship breached the Japanese territorial water just off the southern prefecture of Kagoshima.

Moriya said the Japanese government expressed “our serious concerns” to China through diplomatic channels on Wednesday.

“China has increasingly expanded and intensified military activities around Japan in recent years,” Moriya said. “We will continue to closely watch Chinese warships’ activity around Japan and the regional waters while ensuring information gathering and vigilance.”

In Beijing, Chinese Foreign Ministry spokesperson Lin Jian defended the activity of his country's warships, saying it “complies with China’s domestic laws and international laws.”

Tokyo had reinforced defenses in southwestern Japan, including remote islands that are considered key to Japan’s defense strategy in the region.

The Soviet-built Liaoning underwent extensive refurbishing before being commissioned in 2012 as China's first aircraft carrier. The country now has two more aircraft carriers, including one fully designed and built in China.