The comments by ambassador to Beijing Ji Jae-Ryong were the first official response by the North to Moon's triumph last week, except for a four-paragraph dispatch by the official Korean Central News Agency two days after the vote.
Moon - who favours engagement with Pyongyang to curb its nuclear ambitions -- won overwhelmingly after his conservative predecessor Park Geun-Hye was ousted over a massive corruption scandal.
"The South Korean people are longing for new politics, new society and new life and the election was a reflection of this popular sentiment," Ji told reporters in Beijing.
"Anyone who pursues selfish interests by following a foreign power and keeping their distance from their compatriots cannot avoid the stern judgement of the people," Ji said.
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It was important for Seoul's leadership to "faithfully abide" by previous North-South agreements, he added,
Moon was part of the South's last liberal government nearly a decade ago, which pursued a "Sunshine policy" of reconciliation and dialogue with the North.
He is widely expected to shift away from Park's approach, and declared at his swearing-in that he would go to Pyongyang "in the right circumstances".
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