As maritime tensions in the Pacific remain, Japan’s defense ministry said on Sunday that a submarine believed to be from China was sighted in waters near its southern islands.

The Japanese navy spotted a submerged vessel heading northwest just outside territorial seas near Amami Oshima island, part of Kagoshima prefecture, on Friday morning, according to a statement from the ministry. In the area, a Chinese warship was also observed.

In recent years, Tokyo has protested about Chinese boats entering its territorial seas and approaching disputed islands. China has frequently expressed its displeasure with US ships passing through disputed regions of the South China Sea in what Washington refers to be “freedom of navigation” displays.

Visiting Vietnam as part of a Southeast Asia trip, senior Chinese diplomat Wang Yi urged the two nations should avoid unilateral measures in the South China Sea that may complicate and escalate conflicts.

Sunday’s announcement said Japan’s Maritime Self-Defense Force identified the vessels in a contiguous zone, which is outside territorial waters where vessels are required to identify themselves. Still, Defense Minister Nobuo Kishi instructed his staff “gather information and maintain vigilant surveillance with a sense of urgency,” the statement said.

Officials at the Chinese embassy could not immediately be reached for comment on Sunday.

The submarine continued underwater westward in the ocean near Yokoate Island, the ministry said.