TOKYO (THE YOMIURI SHIMBUN/ASIA NEWS NETWORK) - In late October, I spent nine days embedded aboard the Kaga, the largest helicopter carrier-type destroyer of the Maritime Self-Defence Force, as the vessel toured the South China Sea on a long-term training deployment.

The South China Sea has been the site of increased manoeuvring between China and the United States.

China's presence is growing as it continues to construct military bases on artificial islands, putting it at odds with the United States and its goal of advancing freedom of navigation in international waters.

Under such tense circumstances, Japan and China have also been probing each other's true intentions in the region.

It was past 5:30 a.m. on Oct. 25.

A silent tension gripped the bridge of the Kaga as it sailed through the South China Sea south of Vietnam.

A Chinese warship, whose silhouette had only just come into view, had closed rapidly and begun to tail the Kaga, settling into a position about 10km from the Kaga's right rear side.

Suddenly, a voice burst through over an international radio channel.

"Japanese Warship 184. This is Chinese Warship 170. Good morning. Nice to meet you. Over."

The Kaga's hull number is 184.

The ship tailing the Kaga was ship number 170 of the Chinese Navy - the Lanzhou, a Luyang II-class guided-missile destroyer, with a full-load displacement of 7,112 tons.

The Lanzhou is said to be equipped with anti-aircraft and antiship missiles, as well as a phased array radar that can acquire multiple targets simultaneously.

It is called the "Chinese Aegis-equipped destroyer."

It is also the ship that had a near miss with an Aegis-equipped U.S. destroyer near an artificial island in the Spratly Islands in late September, coming within about 41 meters of the U.S. destroyer, which is believed to have been conducting freedom of navigation operations.

"We're being hailed. Now responding."

Upon relaying this to their superiors, the Kaga's assistant anti-submarine warfare officer returned the Chinese vessel's hail with much the same phrasing, but the conversation with the Lanzhou ended there.

The Kaga was in the midst of a round-trip to the Indian Ocean as part of the Maritime and Self-Defence Force's (MSDF's) first Indo-Southeast Asia Deployment.

That morning, the Kaga and the Inazuma, an accompanying destroyer, were refueled at sea by the Pecos, a U.S. Navy replenishment oiler.

The refueling was part of the Acquisition and Cross-Servicing Agreement between Japan and the United States.

The Lanzhou closed to about five kilometers away during the about two-hour process, and continued to follow and monitor the MSDF ships until early the next morning without taking additional provocative action.

The refueling took place outside the so-called "nine-dash line," within which China claims to have historic rights to sovereignty.

But the setting symbolises the standoff between China and Japan and the United States, with the former accelerating its maritime expansion and the latter two working in close coordination in the South China Sea.

At night, the game of tag between the MSDF and the Chinese Navy took on a new dimension.

"Screech, screech."

Sometime after 2 a.m. on Oct 26, a sound reminiscent of a whale's song rang out repeatedly through the darkened halls of the ship, waking me.

It was the sound of the Kaga's sonar as it searches for submarines.

About 10 days prior, on the way back from the Indian Ocean, something happened that surprised the crew of the Kaga.

At the entrance to the Strait of Malacca, the Japanese vessel encountered one of the Chinese Navy's submarines, which was cruising at the surface.

It is rare to encounter another country's submarine carrying out covert actions.

A Chinese submarine is an especially rare sight.

The submarine passed through the Strait of Malacca ahead of the Kaga and disappeared into the South China Sea.

The MSDF is believed to have conducted a search for the submarine while heading north through the South China Sea on Oct. 26, in an effort to spot it a second time.

It is believed that in addition to the Kaga's on-board sonar, a sonar device called a sonobuoy was dropped from an SH-60K patrol helicopter, and the search was carried out in cooperation with the Inazuma.

In the early morning on Oct. 26, back on the surface, a smaller Jiangkai II-class frigate vessel began tailing the Kaga in place of the Lanzhou.

Ultimately, the search for the Chinese submarine on Oct. 26 likely had to be stopped, as the MSDF ships were forced to increase their speed in order to get through the South China Sea quickly to avoid a direct hit from Typhoon No. 26, approaching from the east.

The Kaga passed through the Bashi Channel on the night of Oct. 27 and arrived at Okinawa on the morning of Oct. 30, completing its deployment.

The Kaga's two-month deployment was part of the "free and open Indo-Pacific strategy" that the administration of Prime Minister Shinzo Abe has touted.

During that time, the ship made port calls in the Philippines, Indonesia, Sri Lanka, India and Singapore, and took part in separate exercises with seven countries in total - those five nations plus the United States and Britain.

In mid-September, the Japanese government made the first public announcement concerning the MSDF's anti-submarine drills in the South China Sea, which had been occurring for some time.

The announcement was a sign that Japan's efforts to increase its presence in the South China Sea had begun in earnest.

Japan's strategic releases of information are thought to be modeled in part on methods employed by countries such as the United States and Britain.

At present, the MSDF and the Chinese Navy directly communicate with each other under the Code for Unplanned Encounters at Sea (CUES), an international standard that establishes protocols for the purpose of preventing maritime incidents.

During the Kaga's outward voyage, as it headed south through the South China Sea, it used the wording stipulated by CUES to exchange course and speed information with the tailing Chinese ship.

The MSDF's Cort Flotilla 4 Commander Tatsuya Fukuda, 51, who oversaw the deployment, said: "There is a difference in the level of tension between the East China Sea and the South China Sea. In the East China Sea (the site of conflict over the Senkaku Islands), China is more aggressive, while in the South China Sea, it is gentlemanly. We're aware China is also starting to take actions that have a legal basis."

However, in this instance, the MSDF had prepared itself for the possibility of an emergency with China, including by having a legal officer well versed in international law on the crew of the Kaga.

In contrast to its attitude toward the United States, China has taken a calmer approach to Japan because Japan has not approached its artificial islands.

Nonetheless, the MSDF must remain vigilant in the face of China's "smile diplomacy."

The South China Sea is a key sea-lane for Japan.

If it became necessary to take a detour around the South China Sea, the additional time and fuel costs are estimated to be 1½ days and US$120,000 (S$165,216) for travel via the Sunda Strait, and three days and US$240,000 for travel via the Lombok Strait.

Both of these straits can be perilous, with strong tidal currents, sunken ships and shoals.

If either were to see a large increase in marine traffic, chaos is predicted to ensue.

According to Kenji Hongan, 43, a captain for the shipping company NYK Line, a growing number of oil rigs and research vessels of unknown nationality have been sighted on and alongside the regular routes of merchant ships in the South China Sea in recent years, with the rigs' and research vessels' crews frequently instructing other ships in English to "keep five miles (about eight kilometers) away and pass through."

China has a base for nuclear-powered ballistic missile submarines in Sanya on Hainan Island.

China's construction of military footholds in the Spratly Islands in the southern half of the South China Sea is said to also be aimed at making the deeper waters of the northern half of the South China Sea "off-limits" and protecting its nuclear submarine base.

A court of arbitration on the South China Sea handed down a ruling in 2016 that it would not recognise China's claims to "historical rights" to the waters within the nine-dash line.

"China has declared it will not abide by the verdict. But this means that a permanent member of the U.N. Security Council denies the rule of law. In order to establish the rule of law in the South China Sea, China is being called upon to adopt the posture of a responsible power, one that respects judicial decisions," said Shigeki Sakamoto, a professor of international law at Doshisha University.

In the immediate future, it will be necessary to maintain a careful watch over China's movements in and around the South China Sea.

The Yomiuri Shimbun is a member of The Straits Times media partner Asia News Network, an alliance of 23 news media entities.