It was further fueled by those disgruntled by the participation of the North, which has been aggravating regional tensions with its ballistic missile and nuclear programs.
The invite was presented verbally to President Moon Jae-in by Kim's younger sister, Kim Yo Jong, during a historic meeting between North and South Korean officials at Seoul's presidential Blue House.
"In my experience, the Olympic Games does not have organized cheerleading, but yesterday the North Korean cheering squad prepared a lot to cheer on the athletes and they changed their movements to the music and throughout the Games", said organizing committee spokesman Sung Baik You.
South Korea's Unification Ministry has sought to play down the incident, saying the photo was just a "good-looking" man.
Kim's sister, Kim Yo Jong, led a charm offensive during the Games, inviting Moon for a summit in Pyongyang and asking him to play a "leading role" in reuniting the two Koreas after almost seven decades.
The North Korean delegation also included the country's 90-year-old head of state, Kim Yong Nam.
"This demonstrates the type of flexibility the US needs to move forward", said William McKinney, a visiting scholar at the US-Korea Institute of the Johns Hopkins School of Advanced International Studies who has participated in past talks with North Korea. "I don't know", he said.
"I never expected to come here on such short notice to be honest, and I thought it would be unusual and different but it's not", Kim Yo Jong said at a dinner Sunday night before heading home.
In December, US Secretary of State Rex Tillerson's offered to start talks with North Korea without pre-conditions, but the State Department later said there would first have to be a "period of calm" in which Pyongyang suspends testing before any negotiations could begin.
But he rejected any suggestion Sunday that even a temporary warming of relations between the North and South could drive a wedge between Seoul and Washington.
Mattis said it's "too early to say" whether the situation will continue to remain cool after the Olympics. "We can't say right now".
Last year, North Korea conducted dozens of missile launches and its sixth and largest nuclear test in defiance of U.N. Security Council resolutions as it pursues its goal of developing a nuclear-armed missile capable of reaching the United States.
Reunification would be no picnic; it would be South Korea taking on the problems of the north, and very likely after a violent and sudden collapse. The option is predicated on Kim's rationality; that once hit, he would not retaliate in a serious way because to do so would ensure the destruction of his regime, and the collapse of his country, in a war with the USA and its allies.
The Games in South Korea are due to finish on Feb 25. Saturday's game ended in a crushing 8-0 loss to Switzerland. One option is what's come to be known as the "bloody nose" strategy.
He said the Koreas should create an environment so that a summit could take place. It also increases the chances the United States and North Korea will soon begin a process that represents the best hope of preventing a devastating global conflict.
North Korean leader Kim Jong Un deployed a new weapon at the Olympics to fight back against the Trump administration's sanctions and threats of a pre-emptive strike against his nuclear program: his sister.
By also sending a youthful, photogenic individual who would surely draw worldwide attention at the Olympics, Kim might have also been trying to construct a fresher image of the country, particularly in face of US efforts to use the Olympics as an occasion to highlight the North's brutal human rights record. Her picture, often with a bemused half-smile, has been splashed across newspapers. She also delivered a letter inviting Moon to a summit with her brother in Pyongyang, and asked him to play a "leading role" in reuniting the two Koreas after almost seven decades.
The White House did not immediately respond to a request for comment.
Pence told Moon the global community must not repeat the mistakes of the past by giving North Korea concessions in exchange for talking.
Kim Yo-jong is widely regarded as one of her brother's top confidantes and senior officials in the North Korean Government.