North Korea leader Kim Jong-un's sister, Kim Yo-jong, was in attendance alongside South Korea's president Moon Jae-in and North Korea's nominal head of state, Kim Yong-nam.
The two sides stressed on Saturday that they were interested in maintaining movement toward peace.
That was not the case with Moon - either at the games, when he enthusiastically reached up to shake Kim Yo Jong's hand, or at the lunch the next day.
Such a high-level meeting would have been unthinkable even a few months ago, but 2018 has seen an accelerated rapprochement between the two adversaries in the run-up to the Games.
"It is too early to say if. using the Olympics in a way to reduce tensions, if that is going to have any traction once the Olympics are over, we can't say right now", Mattis said.
Moon had previously expressed willingness to travel to Pyongyang, a move likely to be opposed by the USA, as Washington pursues a policy of "maximum pressure".
Joint U.S. -South Korean military exercises have been postponed until after the Olympics, but the U.S. insists that no further delays are possible.
The cheerleading troupe also attended the opening ceremony at Pyeongchang Olympic Stadium on Friday night, where they sat silently throughout the show until the unified Korean team entered the stadium.
The apparent thaw has not been reflected in Washington, however. This new found amity has US Vice President Mike Pence anxious.
On his flight to Alaska on Saturday, the vice president said he left Asia "encouraged that we will continue to work very closely to continue and intensify the maximum pressure campaign" against North Korea.
Pyongyang has ruled out a meeting with the United States delegation during the Winter Games.
Moon has also dismissed a call by Japanese Prime Minister Shinzo Abe to quickly resume the U.S. "And we hope you take the comment as it is".
"I was so deeply moved, moreso than others, and I dream of unification and returning to my homeland", Young Ae Ma said through a translator.
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