Hours after US President Donald Trump confirmed the on-again off-again summit with his North Korean counterpart would go ahead, US Defense Secretary Jim Mattis ruled out any discussion about US troops in the region.
Mr Mattis said the issue of US troops stationed in South Korea will not be "on the table" at the summit.
"That issue is not on the table here in Singapore on the 12th (of June), nor should it be," he said at the Shangri-La Dialogue, a security summit in Singapore, referring to the scheduled date of the Trump-Kim meeting.
There are currently some 28,500 US forces based in the South.
Trump said Friday he will meet Kim for the historic summit as originally scheduled after extraordinary Oval Office talks with a top envoy from Pyongyang.
Trump told reporters that denuclearisation - and a formal end to the decades-old Korean war - would be on the table in Singapore.
A US jet lands during South Korea and US joint military exercises conducted last month. Source: Yonhap
However, Mattis stressed that "any discussion about the number of US troops in the Republic of Korea is subject to... the Republic of Korea's invitation to have them there, and the discussions between the United States and the Republic of Korea, separate and distinct from the negotiations that are going on with DPRK (North Korea).
"That issue will not come up in the discussion with DPRK."
But he added: "Obviously if the diplomats can do their work, if we can reduce the threat, if we can restore confidence-building measures with something verifiable, then, of course, these kinds of issues can come up subsequently between (South Korea and the US)."
Last month, South Korean President Moon Jae-in dismissed claims that US troops stationed in the country - based on Seoul's alliance with Washington - would have to leave if a peace treaty was signed with the North.
Ending the war
US President Donald Trump has also warned that he did not expect to immediately sign a deal to bring a halt to the North's nuclear program.
"I never said it goes in one meeting. I think it's going to be a process, but the relationships are building and that's very positive," he said, after waving farewell to the North Korean envoy, Kim's right-hand man.
Trump said they had discussed formally ending the Korean War, which has been largely frozen since an armistice ended hostilities, but not the underlying conflict, in 1953. Since then, there have been occasional clashes on the divided peninsula.
"We talked about it. We talked about ending the war," Trump said.
"Historically it's very important, but we'll see. We did discuss that, the ending of the Korean War. Can you believe we're talking about the ending of the Korean War?"
Washington is determined that Kim should agree to what US officials call the "complete, verifiable and irreversible" end of North Korea's nuclear weapons and intercontinental missile programs.
Kim says he is committed to "denuclearization" in some form, but he is expected to demand security guarantees - one of which could be a formal end to the conflict with the US and South Korea.
Most expert observers are sceptical that even an unprecedented summit between the two leaders can lead to a rapid breakthrough, and Trump admitted it would be a long and difficult process.
President Donald Trump says the June 12 summit is on after meeting North Korean envoy Kim Yong-chol. Source: AAP
"We're not going to go in and sign something on June 12. We never were. I told him today, 'Take your time'," he said, adding nevertheless that he expects "a really positive result in the end."
Kim Yong-chol, vice chairman of the ruling party executive and the most senior North Korean to visit the United States in 18 years, spent almost 90 minutes in the Oval Office.
Afterwards, Trump and US Secretary of State Mike Pompeo walked the North's small delegation to their waiting cars, smiling and shaking hands in front of the media before the motorcade pulled away.
Security guarantees
North Korean officials said Kim Yong-chol was expected to return to Pyongyang shortly. Meanwhile, discussions between US and North Korean officials continue in Singapore and in the Demilitarized Zone between North and South Korea.
On Thursday, Kim Jong-un told Russia's Foreign Minister Sergei Lavrov that his commitment to denuclearization remains "unchanged and consistent and fixed," but experts warn he will seek concessions from Washington.
In addition to an end to the war, he is likely to seek international recognition as well as guarantees against any strike by the US forces stationed in South Korea.
As expected, Kim Yong-chol handed Trump a letter from Kim that may clear up some of the questions. The US leader said the missive was "very nice" - but then admitted he had not yet read it. An aide later confirmed he did after the talks.
The Oval Office talks and letter delivery came only a week after Trump threatened to consign the entire process to history, abruptly cancelling the summit in a sharply worded letter, only to revive preparations shortly afterwards.
Trump said that, after Friday's talks, the parties are "totally over that and now we're going to deal and we're going to really start a process."
Since the short-lived boycott threat, diplomats from both countries have conducted an intense flurry of talks, culminating this week when Pompeo sat down in New York with Kim's envoy.
'Their decision'
Pompeo said on Thursday that, after what have now been two meetings with Kim Jong Un and three with Kim Yong-chol, he believes the North is at least ready to consider addressing US demands for denuclearization.
"I believe they are contemplating a path forward. They can make a strategic shift. One that their country has not been prepared to make before. This will obviously be their decision," he said.
Top North Korean official Kim Yong-chol (left), President Donald Trump, and another North Korean advisor Kim Song-hye pose at the White House. Source: AAP
The flurry of diplomacy has also seen a rapprochement on the Korean peninsula, with the two Koreas holding high-level talks Friday at the border truce village of Panmunjom.
The meeting followed two landmark summits between the leaders of North and South Korea in the last five weeks.
North and South Korea agreed to hold more meetings throughout this month to carry out the agreements reached between their leaders at the April summit, according to a joint statement issued after Friday's talks.
