The North Korean taekwondo athletes arrived South this week and performed at the Pyeongchang Olympic Games opening ceremony on Friday as part of four performances during their nine-day trip.
Kim Yo Jong is the first member of the ruling Kim family bearing the bloodline of the sacred Mount Paektu, a centrepiece of the North's idolisation and propaganda campaign, to cross the border into the South since the 1950-53 Korean War.
Mattis also said he doubted that the sporting rapprochement would undermine US-South Korean ties.
It then described the North Korean dictator's sister, Kim Yo Jong, as having "struck a chord with the public", and even termed her "North Korea's Ivanka Trump". "Moon should be aware that he has to urge denuclearisation to North Korea by himself, not depending on dialogue between the US and North Korea", an editorial in the Yomiuri Newspaper read.
"The point is, no pressure comes off until they are actually doing something that the alliance believes represents a meaningful step toward denuclearization", Pence was quoted on Sunday as saying.
Despite Pence's hardline stance and words, he has also signaled the United States is possibly open to negotiations when he said in regards to the North Korean officials, "I haven't requested any meeting".
"The vice-president is grateful that President Moon reaffirmed his strong commitment to the global maximum pressure campaign and for his support for continued sanctions", Pence spokeswoman Alyssa Farah said when asked about the development.
Although Kim Yo Jong ranks diplomatically lower than Kim Yong Nam, as Kim Jong Un's sister she is considered more important in North Korea. Moon's chief of staff, Yim Jong-suk, held a private farewell dinner for the delegation at a hotel in Seoul on Sunday.
South Korean officials have said Moon was invited to Pyongyang to speak with Kim Jong Un during the talks and lunch that Moon hosted at the presidential Blue House in Seoul on Saturday.
The visit is part of an agreement between the International Olympic Committee and both North Korea and South Korea, according to a source.
Wearing a black coat and with her hair pulled back, she presents a marked contrast to the ceremonial head of North Korea's government - the often expressionless Kim Yong Nam, 90.
Moon replied: "Our meeting was precious".
However, South Korea's main opposition party warned any talks between the two Koreas where the scrapping of North Korea's nuclear programme was not a precondition would only "benefit the enemy".
The CNN article came shortly after many liberals cheered Kim Yo Jong for giving Vice President Mike Pence a "deadly side eye" during the Olympic games. About 800 conservative protesters gathered in central Seoul to tear up photos of Kim Jong Un and condemn Moon for being too soft on North Korea. "It is the fake peace, this is 'peace Olympics'".
Moon and Kim Yo Jong had their first face-to-face encounter at the opening ceremony.
Pence and Moon took two very different diplomatic approaches to the North Korean delegation's attendance at the games in Pyeongchang. "It will be hard for the allies to resume their military exercises during summit planning". Moon had replied, "Let's create the environment for that to be able to happen", according to the presidential Blue House.
The hardline US policy is at odds with the more conciliatory approach towards the North favoured by South Korea.
The ladies, all in their late teens or early 20s, are said to be handpicked from elite universities and undergo strict background checks.
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