Japan's defense ministry said Sunday that they detected a submarine believed to be Chinese off a southern island, heightening the country’s caution levels in the East China Sea as China increases its military activities.

The submarine remained submerged, but the ministry said in a statement that it believes the submarine is Chinese because a Chinese Luyang III-class guided-missile destroyer is near the submarine.

The submarine moved northwest off the eastern coast of the Amamioshima Island, about 700 kilometers (420 miles) northeast of the disputed East China Sea islands controlled by Japan but also claimed by Beijing, the ministry said.

The submarine on Sunday morning was heading west in the East China Sea.

Neither the submarine nor the ship entered Japanese territorial water. Under international law, submarines passing off the coast of another country are required to surface and show a national flag inside territorial waters.

Japan's Maritime Self-Defense Force sent three reconnaissance aircraft and two destroyers to the area for early warning and information gathering to analyze China’s intentions.

A submarine believed to be Chinese also was spotted in the area in June 2020.

China has defended its maritime activities and says it has the right to defend its sovereignty, security and development interests.

Japan, alarmed by China's growing naval activities in the East and South China seas, has been stepping up defense in the country’s southwestern regions and islands north of the disputed islands.

Tokyo says it opposes China's unilateral attempts to change the status quo in the region, and regularly protests the Chinese coast guard’s growing presence near the disputed islands. Japanese officials say Chinese vessels routinely violate Japanese territorial waters around the islands, sometimes threatening fishing boats.