Mattis' remarks come after South Korean President Moon Jae-in was presented a formal invitation to visit North Korea, potentially setting up the first meeting of Korean leaders since 2007.
The 28-year-old is a crucial part of the North's Games delegation led by Kim Yong Nam, North Korea's nominal head of state.
North Korean leader Kim Jong Un invited the South's President Moon Jae-in for a summit in Pyongyang today, Seoul said, even as the United States warned against falling for Pyongyang's Olympic charm offensive.
Moon asked the North Korean delegation during Saturday's meeting to engage in dialogue with Washington "at an early date", the Blue House said.
The Trump Administration, represented by Vice President Mike Pence in PyeongChang, has warned the South Koreans about falling for North Korea's "charm offensive" at the Winter Games. Kim Jong-un's wife was a member of the squad that came in 2005.
But as tensions rose between the North and Washington, the Games triggered a rapid reconciliation between the two Koreas, who are still technically at war.
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That part is yet to be determined, as politicians, analysts and curious citizens debate whether the North Koreans' amicable presence at these Olympics is legitimate or simply the latest version of a "charm offensive" from the dictatorship, a calculated move created to engender global support and separate South Korea from its alliance with the United States.
Moon's desire to engage North Korea was in contrast to his USA ally.
Her brother - the third generation of her family to rule the isolated and impoverished North - will be pleased with her worldwide diplomatic debut, said Yang Moo-jin, professor at the University of North Korean Studies at Seoul.
Their stories demonstrate the fear the North Korean regime inspires in the American mind, even as South Koreans who live within miles of the contentious border every day greet the provocations and threats of nuclear war with shrugs.
North Korea's state news agency said on Sunday its delegation sent to the Winter Olympics held "frank and candid" talks with South Korean President Moon Jae-in but there was no mention of the North's invitation to Moon to Pyongyang for a summit.
Moon also urged North Korea to talk to the United States, his office said.
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Componentes da escola ( inclusive alguns artistas ), claramente emocionados, usaram camisas com nome e rosto de Arlindo Cruz. A escola é uma das mais tradicionais do Carnaval do Rio de Janeiro , já tendo conquistado nove títulos na história.
So as pleasant as that lunch between North and South Korean leadership may have been, it won't mean much until diplomats from the USA settle in for some of the local delicacies.
North and South Korea competed together for the first time at an Olympics on Saturday as the first of 102 gold medals were decided against a fast-moving backdrop of diplomatic manoeuvring.
Muller scored her fourth early in the second period.
The Koreans are rank outsiders in the tournament for which they only qualified by virtue of being the host nation and are likely to suffer defeat against the Swiss.
However, the result mattered less than the political symbolism as around 200 of North Korea's famed "army of beauties" charmed a crowd of 3,600 besotted locals with nostalgic oldies - and even broke out a Mexican wave. Moon and Kim Yong Nam planned to return to the Games venue to watch the joint Korean women's ice hockey team - the first ever combined team at the Olympics - take on Switzerland, the Blue House said.
Moon made it clear that the issue was an internal affair and that it was inappropriate for Abe to have brought up the matter, Reuters reported.
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International Olympic Committee President Thomas Bach called it "a very emotional moment", one that gave him "goosebumps". Ms Kim, who is though to be about 30 years old, was promoted to the powerful politburo last year.