North Korea on Wednesday said it might not attend the summit if Washington continued to demand it unilaterally abandon its nuclear arsenal.
USA and North Korean officials are expected to discuss how much of the stockpile can be taken out of North Korea within the six-month timetable during working-level talks prior to the summit.
Planned talks between President Donald Trump and North Korean leader Kim Jong-un have also been thrown into doubt. Trump said the two cases are very different. South Korea believes North Korea remains committed to improving relations despite strongly criticizing Seoul over ongoing U.S.
Trump said: "We're willing to do a lot".
As CBS News has reported, the USA has said it isn't seeking regime change in North Korea.
Trump put North Korea back on the list of state sponsors in November, after a nine-year hiatus, allowing the United States to impose more economic sanctions against Pyongyang over its nuclear and missile programs.
Asked about a statement by his national security adviser, John Bolton, who said last weekend that the "Libyan model" might be a suitable framework for denuclearisation, Trump said: "The model, if you look at that model with Gaddafi, that was a total decimation".
Not surprisingly, North Korea bristles at the mention of Libya.
"Nothing has changed on North Korea that we know of", Mr. Trump said.
McMaster was an early Trump supporter in the 2016 presidential campaign, formally backing him ahead of South Carolina's first-in-the-South primary.
He said the North Korean leader was cooperating with trade discussions. "In Libya we decimated that country", Trump said. "That is, we don't fully understand what's in the mind of Kim Jong-un on a variety of issues and we're always trying to get more intelligence".
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Of the 34 suspected cases, two were confirmed, 18 have been classified as probable and 12 remains suspected cases. The outbreak is now confined to the Bikoro, Iboko, and Wangata health zones, in Equateur province.
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Trump says Kim needs to demonstrate the will to reach an agreement.
North Korea ramped up its criticism of South Korea and warned of a "rupture" in ties.
The secretary of state has said North Korea must embrace "complete, verifiable, irreversible denuclearization". "There was no deal to keep Gaddafi".
According to North Korea's official KCNA news agency, Pyongyang's first vice minister of foreign affairs, Kim Kye Gwan, specifically criticized USA national security adviser John Bolton, who has called for the North to quickly give up its nuclear arsenal in a deal that mirrors Libya's abandonment of its weapons of mass destruction.
But Trump said Thursday that nothing has changed with respect to North Korea after the warning from Pyongyang.
"The reality is this could mean a lot of things ranging from 1) nothing, to 2) DPRK [North Korea's] domestic politics to 3) a warning shot about all the U.S. maximum pressure talk to 4) a real change in DPRK intentions", tweeted Vipin Narang, an MIT professor who specializes in nuclear strategy.
"North Korea is talking to us as if nothing happened", Trump told reporters during an extended bilateral meeting between him and NATO's Secretary General. It includes aircraft from across the USA military services.
He noted that "protection will be very powerful", reports the Associated Press.
May 12: North Korea says it will hold a ceremony to dismantle its nuclear test site between May 23 and 25 and foreign journalists will be invited to attend.