The unprecedented presence of an advanced Chinese nuclear stealth submarine in the waters near islets contested by Japan and China in the East China Sea has jeopardised tentative moves to improve bilateral ties.
Japanese Defence Minister Itsunori Onodera yesterday slammed the Chinese action as one that "unilaterally raises tensions". Beijing, however, said the sub last Thursday was simply tracking and monitoring two Japanese naval ships passing through the area.
The Chinese submarine has been identified as a new Shang-class vessel that is 110m long, with a displacement of 6,100 tons. It can be equipped with torpedoes and cruise missiles that have a longer range than the ones found on conventional submarines.
In a separate incident yesterday, three China Coast Guard patrol vessels entered the territorial waters around the islets known as Senkaku in Japan and Diaoyu in China. It was the second such incident this year, with the first occurring last Sunday.
Tokyo has lodged protests against these "incursions", prompting Beijing to retort that it does not accept these representations as it considers the islets a part of Chinese territory.
"We urge Japan to stop stirring up trouble on the Diaoyu islands issue," Chinese Foreign Ministry spokesman Lu Kang said in Beijing yesterday, in response to questions posed by Japanese public broadcaster NHK.
Earlier yesterday, at a news conference in Tokyo, Japanese Chief Cabinet Secretary Yoshihide Suga described the Senkaku islands as being "Japan's inherent territory, legally and historically". He also said the spate of incidents recently was "extremely regrettable".
Last week, Mr Suga reaffirmed that Japan would "resolutely defend its land, territorial waters and air space", while handling the situation "firmly and calmly".
Waters around the uninhabited islets, which are administered by Japan, are said to be rich in oil and natural resources. The islets have repeatedly caused tensions to flare up between China and Japan, and the latest incidents have come just as ties were getting better. Chinese President Xi Jinping and Japanese Prime Minister Shinzo Abe vowed a "fresh start" to relations in rare bilateral talks last November.
Last Thursday's incident marks the first time since June 2016 that a Chinese military vessel has entered the so-called "contiguous zone" around the disputed islets. It also marks the first time a nuclear submarine has entered the area.
Mr Onodera, speaking to reporters yesterday, said: "Such nuclear-powered submarines are difficult to detect because they can remain far beneath the surface for extended lengths of time. We'll keep our guard up to respond swiftly if a similar incident happens again."
Mr Suga yesterday said there was an urgent need to step up bilateral efforts, and set up an air and maritime communication mechanism that could help avoid accidental conflict in the East China Sea. Territorial waters refer to an area extending 12 nautical miles from the coast of a country, while the "contiguous zone" refers to the band of water in an area between 12 and 24 nautical miles from land.
The submarine's movement through the contiguous zone does not contravene international law. Beijing's Ministry of National Defence voiced "strong discontent with Japanese efforts to sensationalise a legitimate action by the Chinese navy".
University of Tokyo professor Shin Kawashima, who studies Sino-Japanese ties, said the entry of a nuclear submarine into waters near the disputed islands marked a "new phase in Chinese escalation".
He said: "China is sending a message to Japan, that while on the one hand, it is willing to improve bilateral ties, on the other, it will not loosen its stance on territorial and security issues in the East China Sea."
Emeritus professor Shinya Murase, from Tokyo's Sophia University - who is also a visiting professor at Peking University and a member of the United Nations International Law Commission - said China ought to resolve the issue of the disputed islets by peaceful means, such as taking its claims to the International Court of Justice.