A US$5 billion (S$6.81 billion) demand to meet the cost of hosting American troops, and tensions between Seoul and Tokyo that threaten to undercut regional cooperation are set to top the agenda when senior US defence officials visit South Korea this week.
US President Donald Trump's insistence Seoul take on a greater share of the cost of American military presence as deterrence against North Korea has tested South Korea's confidence in the security alliance with Washington.
Mr Trump has floated the idea of pulling US troops from the Korean peninsula, which remains in a technical state of war under a truce that suspended the 1950-53 Korean War.
A South Korean lawmaker said last week that US officials demanded up to US$5 billion a year, more than five times what Seoul agreed to pay this year under a one-year deal, for stationing the 28,500 US troops.
US officials have not publicly confirmed the number, but Mr Trump has previously said the US military presence in and around South Korea was "US$5 billion worth of protection".
General Mark Milley, chairman of the US Joints Chief of Staff, said the American public needed an explanation why "very rich and wealthy" South Korea and Japan cannot defend themselves and why US soldiers were deployed there.
Gen Milley, who was speaking to reporters en route to Tokyo on Sunday, arrives in Seoul on Wednesday for the annual Military Committee Meeting.
Secretary of Defence Mark Esper will visit from Thursday for the Security Consultative Meeting with South Korean Defence Minister Jeong Kyeong-doo.
Mr Randall Schriver, assistant defence secretary and Mr Esper's top Asia policy advisor, said the secretary did not intend to negotiate burden sharing, a job for diplomats, but he would emphasise US interests.
"They have to be willing to pick up a larger share of the burden, as the president has emphasised globally, not just related to South Korea," Mr Schriver told a small group of reporters ahead of the trip.
Mr Trump has similarly accused allies including Japan, Germany and Nato of not shouldering their fair share of defence costs.
Separate negotiations for new defence cost-sharing deals between the United States and all three are set to start next year.
South Korean lawmakers have criticised what they called"unacceptable, disappointing" US demands.
Some progressive groups in South Korea have called for a fundamental shift in the 70-year alliance with the United States, including a withdrawal or drastic reduction of US troops.
A survey by the government-affiliated Korea Institute for National Unification released last week showed 96 per cent of South Koreans were against paying more for the US military presence.
"US demands would get more reasonable as negotiations progress, after raising alarm with extremely high numbers," said Mr Shin Beom-chul, a senior fellow at the Asan Institute for Policy Studies in Seoul.
"But there's real pressure from Trump, and even if it goes down from US$5 billion to US$2 billion, it's still a tremendous burden on the South Korean administration."
Mr Esper and Gen Milley are also expected to step up pressure on South Korea to reverse its decision to end an intelligence-sharing agreement with Japan amid a spiralling diplomatic and trade feud.
The pact, called Gsomia, or the General Security of Military Information Agreement, is set to expire next week after Seoul decided not to renew it following Tokyo's imposition of export controls on South Korea.
Washington has criticised the move, seeing the deal as vital to three-way cooperation in fending off North Korea's nuclear and missile threats.
A spokesman at South Korea's foreign ministry reiterated Seoul is willing to reconsider the decision if Japan withdraws its trade regulations.
Gen Milley said Seoul and Tokyo should "get past some of these friction points" as those only benefit North Korea and China.
