Moon said that “under the regional cooperation principles of openness, inclusiveness and transparency, we have agreed to put forth harmonious cooperation between Korea’s new Southern policy and the United States’ Indo-Pacific strategy”.

Seoul’s “new Southern policy” was announced in 2017 after Beijing instituted unofficial sanctions on the nation – including a boycott of South Korean tourism – in response to Seoul’s decision to allow the US to deploy an anti-missile system in the country.

Beijing had protested the US’ move, saying it would allow the American government to look deep into the mainland and monitor China’s military facilities.

The new South Korean policy was widely viewed as an extension of its effort to lessen its reliance on a single country – China – and diversify its economic market by building deeper ties with Southeast and Southwest Asian countries.

South Korean firms had been exiting China to avoid the political and economic challenges that complicated the countries’ relationship amid the US-China trade war and the American military’s deployment of the anti-missile Terminal High Altitude Area Defence (THAAD) system in South Korea.

But Seoul had played down its China-exit strategy and until this week had never explicitly linked it to the US’ Indo-Pacific game plan – possibly involving South Korea’s military presence in the South China Sea.

Seoul has actively responded to Washington’s calls to support global security operations in Vietnam, Saudi Arabia, Somalia, East Timor, Afghanistan, Iraq, the Gulf of Aden, Lebanon, South Sudan and the UAE.

But in the South China Sea, it has maintained an ambiguous presence.

Since Seoul needs Beijing’s support for its effort to reunite with North Korea and end the countries’ seven-decade estrangement, it has tried to avoid getting involved in military conflicts that could provoke Beijing’s ire.

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But it has faced “tremendous pressure” from Washington, as it strives to keep up relations with both the US and China, according to multiple South Korean and US diplomatic sources.

Timothy Heath, senior international defence researcher at Rand Corporation, a California-based think tank, said Seoul’s decision to throw its support behind the Indo-Pacific strategy may have been “influenced in part by pressure by the White House for burden sharing”.

“With Trump taking steps to ease tensions by meeting with North Korean leader Kim Jong-un, Washington may have asked Seoul to do more to support US efforts to promote stability and balance in the region,” Heath said.

“Seoul may have also calculated that Beijing is in a weak position to retaliate, due to the ongoing trade war with the US.”

Analysts said Seoul’s participation in the strategy would significantly increase the operational capability of the US and its allies in the region, especially in the South China Sea.

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The US and its allies have been sending military vessels to the South China Sea to counterbalance China’s deployment of troops and weaponry in the contested waters and blunt its enhancing of radar systems and building up piers to house navy vessels, among other military activity.

According to the 2019 Global Firepower Index, a ranking of world military strength by country, South Korea is the world’s seventh strongest military power after the US, Russia, China, India, France and Japan.

The South Korean navy has about 70,000 regular personnel and 150 commissioned ships.

The naval aviation force consists of about 70 fixed-wing and rotary-wing aircraft, while the marine corps has about 300 tracked vehicles, including assault amphibious vehicles.

Seoul announced in October a plan to expand its maritime forces into a “blue-water” navy – capable of operating globally, across oceans – to safeguard its strategic interests amid China’s expansion in the South China Sea.

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The plan would “expand the navy’s operational area into far seas, to secure maritime traffic routes, and ensure the free maritime activities and the safety of our citizens”, the South Korean navy reported to parliament.

Bruce Bennett, a senior defence researcher at the Rand Corporation, said it was important to remember that the defence treaty between the US and South Korea was “a mutual defence treaty – it is not just about defending South Korea”.

For decades, Bennett said, the US had “provided forces and made commitments to defend the South against North Korean hostility”.

China, he continued, “will not be happy with the South if it defies China’s claims in the South China Sea … [But] it appears that over time, China wants to establish a position of dominance over its neighbours, hoping to be able to dictate how its neighbours act”.

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If the South wished to maintain independence from Chinese control, Bennett said, “it must be prepared to stand up to Chinese assertions of dominance.

“Otherwise, at some point China will likely insist that South Korea pay China for the right to transit the South China Sea and assert other forms of control.”

Zhao Tong, a fellow at the Carnegie-Tsinghua Centre for Global Policy, said South Korea has increasingly got caught in the middle of the Washington-Beijing power competition.

When South Korea deployed THAAD, Beijing retaliated with its unofficial sanctions, including halting the Chinese expansion plans of the South Korean conglomerate Lotte Corporation, he said. China also banned South Korean TV shows and boycotted South Korean tourism.

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Zhao said Seoul’s “rhetorical support for the Indo-Pacific strategy” showed that South Korea can be expected to rely on the US for security support for the foreseeable future.

“But at the same time, South Korea cannot afford to antagonise China under whose shadow it has to live,” he said. “So far, South Korea has taken care not to deliberately provoke China in the South China Sea and it may continue to do so in the future.”

After Seoul announced it would support the US Indo-Pacific strategy, Washington released a statement welcoming the move, further cementing South Korea’s involvement.

The US urged Seoul to engage in “meaningful cooperation” in “maritime” affairs.

“Both sides reaffirmed that the strong US-ROK alliance is the linchpin of peace and security in the Indo-Pacific,” the US State Department said on Tuesday.

But experts said South Korea’s South China Sea presence was unlikely to be large in the short term, and its involvement would be mainly symbolic.

Rand Corporation’s Heath said “the US is mainly interested in the political and symbolic importance of South Korea’s support for international freedoms in the South China Sea”.

“Occasional South Korean participation in patrols could help serve that goal,” the defence researcher said.

“South Korea may also carry out diplomatic and political statements and activities in support of a free and open South China Sea,” giving the US a “symbolic victory”, Heath said.

“China may protest diplomatically, but it has a strong incentive to treat South Korean Navy ships gently.

“Missteps that resulted in a crisis or dangerous situation would only drive South Korea and Japan closer together and encourage a stronger US-Japan-ROK cooperation,” he said.