BEIJING -- A Chinese submarine was recently detected in the contiguous zone around the Japan-administered Senkaku Islands in the East China Sea.

The incident raised concerns on both sides. Japan was made aware that China is fully intent on continuing its maritime expansion while nominally working to improve ties with Japan. China, on its side, is facing domestic criticism for being easily detected by Japanese sonar.

In the afternoon of Jan. 10, a Japanese Maritime Self-Defense Force P-3C patrol plane dropped a sonar buoy into waters near Miyako Island in Okinawa Prefecture after spotting a submarine cruising underwater in a northwesterly direction.

The device detected the sound of the submarine's engine and the MSDF destroyer Onami began tailing it. On Jan. 12, it was confirmed that the vessel belonged to the Chinese navy.

This marks the first recorded presence of a Chinese submarine in the waters.

The vessel cruised in the direction of the uninhabited Senkaku Islands, which have long been a source of friction between Japan and China. Beijing claims the islands and calls them Diaoyu.

Another destroyer, the Oyodo, joined the Onami in following the submarine, which proceeded to enter the islands' contiguous zone in the morning of Jan. 11. The MSDF warned the submarine that it had entered the zone, but received no response.

When the Japanese destroyers made their way into the zone, they were followed by a Chinese frigate that had been sailing nearby.

The two Chinese vessels moved out of the contiguous zone a few hours later, with the submarine, identified as a Shang-class nuclear-powered attack submarine, resurfacing in the East China Sea on Jan. 12 and sailing under a Chinese flag.

"Is China's nuclear attack submarine too easy to detect?" the South China Morning Post wrote in an opinion piece on Jan 28. "The early and long exposure of its underwater trajectory, according to military experts, suggests the vessel is not as quiet as it should be," the paper wrote.

The United Nations Convention on the Law of the Sea defines a contiguous zone as a band of water from the outer edge of the territorial sea to up to 24 nautical miles (roughly 44 km) from the coastal baseline at its lowest ebb.

Freedom of navigation is granted through a contiguous zone, just as it is on the high seas. But Japan and China have repeatedly faced each other off over the zone around the Senkakus.

Beijing blamed Tokyo for the incident. The Chinese Ministry of National Defense said the submarine and frigate had entered the zone to tail the Japanese destroyers.

When an MSDF vessel sailed into the zone to follow a Russian warship in 2016, a Chinese naval ship subsequently entered it. China, it seems, intends to tail any Japanese vessels entering the zone.

See also

Chinese submarine near Senkakus ignored warnings, Japan says

China and Japan still groping their way to rapprochement

In response to The Nikkei's inquiry, the ministry, which usually sends written answers, replied over the phone that it took the position already announced by China. It has released an official report on the actions of the frigate but has yet to do so for the submarine.

A Japanese Defense Ministry official said the route taken by the submarine indicated it was performing an exercise to measure water depth. In other words, sailing into the contiguous zone was a deliberate act, according to the official.

A person close to the Chinese military said the submarine had barely entered the contiguous zone during its drill, and done so out of carelessness -- adding that Japan and China had hardened their stances toward each other out of mutual distrust.

Opinions are also divided on why the vessel raised its flag on resurfacing. Some say it was a show of force; others brushed it aside as standard procedure for a submarine in shallow waters.

Military sources in Beijing said the navy had acted within the scope of President Xi Jinping's stated policy.

At the Chinese Communist Party's national congress last autumn, Xi declared that China would protect its territory and sovereignty while pursuing stability in the international sphere. As far as relations with Japan are concerned, that implies Beijing will seek to improve ties with Tokyo while continuing its maritime expansion.

The Chinese military appears to have concluded that its latest actions, which were not in breach of international maritime law, would not be detrimental to efforts to improve bilateral ties.

Xi has been addressing discipline within the military of late. On Jan. 3, he implored all the country's armed forces to act as a cohesive unit. Any deviation from Xi's policy would likely be met with severe punishment.

While it is unlikely that the military would deliberately violate international law, acts that skirt the law are far from unimaginable.

In the process of going from being a "big" country to a "strong" one, China appears set to continue its maritime expansion, potentially undermining Japan's control over the Senkaku Islands.

With the 40th anniversary of the Japan-China treaty of peace and friendship coming up, 2018 is a milestone for relations between the two countries. Unfortunately, the year appears to have got off to a shaky start.