In posts on the Truth Social platform, Trump said defence cost-sharing would be part of "one-stop shopping" negotiations with Seoul, and raised the issue of the defence burden during a visit by Japanese officials to Washington this week.

Japan hosts about 50,000 U.S. troops and South Korea 28,500. Both nations rely on the U.S. nuclear umbrella for protection against China, Russia and North Korea, and are seen as crucial for the U.S. military's ability to project power and influence around the region.

Trump has previously suggested he could withdraw the U.S. forces if the countries don't pay up, and during his first term, demanded billions of dollars more.

On Wednesday, South Korea's first vice foreign minister Kim Hong-kyun told parliament that while Washington had not formally proposed renegotiating their Special Measures Agreement (SMA) under which South Korea supports U.S. troops stationed there, Seoul is preparing for various scenarios.

Finance Minister Choi Sang-mok said this week that cost sharing is not up for review.

Tokyo views the issue of defence spending as separate from tariffs, a Japanese government official told Reuters. "These originally are separate issues," the official said, suggesting defence costs should not be part of the tariff negotiations.

The Pentagon and the State Department referred questions to the White House, which did not respond.

Reopening defence talks is a "deliberate pressure tactic" by Trump, said Danny Russel at the Asia Society Policy Institute.

"Trump has made clear he sees alliance relationships as transactional and is determined to extract an economic payoff commensurate with the value of the U.S. defence umbrella," he said.

Shortly before last year's U.S. election, South Korea and the outgoing administration of then-U.S. President Joe Biden hurried to sign a new, five-year SMA under which Seoul would raise its contribution toward the upkeep of U.S. troops in the country by 8.3% to $1.47 billion in the first year, with later increases linked to the consumer price index.

That gamble appears to have failed, however, said one South Korean security official who spoke on condition of anonymity since they were not authorised to discuss the issue.

KEY U.S. ALLIES

The official said it was the Biden team that pushed for the early negotiation, and that it sparked a debate within the South Korean government, which ultimately decided it risked alienating Washington if it refused.

"Now, if we have to negotiate, we are doing it from a higher amount than if we had waited," the official said, but added that the current South Korean leadership is right to say that cost-sharing is not up for review.

South Korea has a caretaker government after Yoon Suk Yeol was impeached and removed from the presidency this month following a brief attempt to impose martial law in December. A new presidential election will be held on June 3.

Japan has the biggest overseas deployment of U.S. troops globally, squadrons of fighter jets and Washington's only forward-deployed aircraft carrier strike group

Under an agreement negotiated in 2022 and set to expire in March 2027, the average annual cost Japan bears towards the upkeep of U.S. troops amounts to around 211 billion yen ($1.48 billion), according to Japan's foreign ministry.

The U.S. troops in South Korea, along with warplanes and armoured fighting vehicles, are there as a legacy of the 1950-1953 Korean War.

"Seoul renegotiated the SMA a year early in order to lock in more beneficial terms prior to Trump’s re-election," said Bruce Klingner, a former CIA analyst now with the U.S.-based Heritage Center.

"Having the defence agreement re-opened and linked to broader economic and trade issues will exacerbate South Korean concerns of the economic concessions it may need to make as well as the continued viability of the U.S. defence commitment."

Questions over the ability, or willingness, of the United States to protect South Korea from nuclear-armed North Korea has sparked new calls for Seoul to prepare to develop its own nuclear weapons.

Analysts said talks with the Trump administration are further complicated by disputes over facts.

Klingner said that just as Trump misquoted South Korea's effective tariff rate in his speech to Congress, his social media post erroneously claimed that the cost-sharing payments had begun under his first term and had been terminated by Biden.

The SMAs began in 1991 and the one signed last year was the 12th such agreement between the United States and South Korea.

In testimony to the U.S. Congress last week, the commander of U.S. Forces Korea as well as the general in charge of all U.S. troops in the Indo-Pacific region praised South Korea's contributions to defence budgets, and its large purchases of American weapons and warplanes.

"These strategic investments underscore the enduring partnership between our two nations," said General Xavier Brunson, commander of USFK.