There are renewed tensions in the East China Sea after a Chinese attack submarine was detected last Thursday operating in the so-called contiguous zone around the Japanese-controlled Senkaku Islands, which China claims as its territory and calls the Diaoyu Islands.

The contiguous zone is a 12-nautical-mile band of ocean that is beyond a nation's territorial waters, which extend 12 nautical miles from a coastline. The Chinese vessel was also operating well within the 200-nautical-mile Exclusive Economic Zone (EEZ) around the uninhabited archipelago.

On Monday, Japanese Defense Minister Itsunori Onodera said in a press conference in Tokyo that "operating a submerged submarine close to another country's territory goes against the norms of international rules."

"We are seriously concerned over acts that unilaterally raise tensions," he said. "We will keep our guard up and respond swiftly if a similar incident happens."

Chinese Ambassador Cheng Yonghua was summoned to the Foreign Ministry in Tokyo last Friday and received an official complaint.

Later in the day, a spokesman for the Chinese Foreign Ministry dismissed the Japanese complaint, repeating Beijing's claim that the islands are an inherent part of China's territory.

China's growing naval ambition

A Chinese naval formation centered around the aircraft carrier Liaoning in January 2017

The 110-meter "Shang-class" submarine has a longer dive range than other vessels in the Chinese navy and is armed with torpedoes and anti-ship missiles with a maximum range of 40 kilometers (25 miles). The submarine was detected by destroyers of Japan's Maritime Self-Defense Forces and confirmed its identity after surfacing and running a Chinese flag up its mast.

Read More: Japan protests after spotting Chinese ships near disputed islets

The intrusion by the Chinese submarine is not the first, with Japanese forces detecting similar operations within its contiguous zone in 2011, 2013, 2014 and 2016. It is likely that other approaches by submerged vessels were not detected.

Analysts have suggested that China's navy is attempting to map subsurface features in order to determine the safest routes for its submarines to follow when they want to sortie into the Pacific Ocean without being detected.

Diplomatically disconcerting

This latest incident is being viewed with indignation by Tokyo because it comes at a time whenthe two governments are making efforts to build bridges after a decade of barely disguised hostility and with the specter of North Korea's weapons program hanging over the region.

Tokyo feels that Beijing's actions are undermining attempts to improve bilateral relations and that the two governments need to show a united front to bring Pyongyang to heel.

One of the disputed Senkaku islands in the East China Sea claimed by Japan and China

"This is clearly not the first such intrusion by the Chinese navy, but I see it as Beijing making a statement that it will not simply accept the status quo in the region, that it has ambitions for a blue water navy and that it will not allow Japan to stand in its way," Jeff Kingston, director of Asian Studies at the Japan campus of Temple University, told DW.

"There has been a lot of talk in recent months about a 'thaw' in the two governments' relations, but it seems to me that territorial and historical issues are fundamental reasons to doubt that can proceed," he said.

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"Both sides are keen to ramp up trade relations and celebrated the 45th anniversary of normalized diplomatic relations late last year, but I feel there is too much baggage to the relationship," he added.

"China has hegemonic ambitions in this region, it realizes that time is on its side and that it will one day in the not too distant future be the predominant power here," Kingston said, adding that Tokyo is right to protest but will most likely be ignored by Beijing.

Yoichi Shimada, a professor of international relations at Fukui Prefectural University, believes Tokyo's anger is "fully justified."

"At a time when there is so much tension on the Korean Peninsula and the international community needs to be focusing on dealing with that situation in order to isolate the regime of Kim Jong Un, we see that not only is China not doing enough on North Korea but is also making these very provocative moves against Japanese territory," he told DW.

Land seizures 'second nature'

Looking at how Beijing has in the past aggressively sought to extend its borders and territory, Shimada points to land grabs on the Indian border and the occupation and militarization of disputed atolls in the South China Sea. The tactic of claiming other nations' land is "second nature," he said.

"And, in response, Japan has no choice but to strengthen its own military capabilities in order to defend our territory," he said.

The timing of the announcement may have been a coincidence, but the Japanese Coast Guard announced on January 14 that it is considering constructing as many as four new bases across Japan large enough to accommodate a new class of patrol ships.

Read more: 'New Silk Road' and China's hegemonic ambitions

Two of the new bases are likely to be in Okinawa Prefecture — which includes the Senkaku/Diaoyu Islands — and a third will be in nearby southern Kyushu. The fourth new facility is likely to be on the north coast of Japan, facing North Korea across the Sea of Japan.

Work on the new facilities is due to start in 2019 and will enable modern new patrol vessels to conduct longer sea patrols closer to the disputed islands, officials said.