SEOUL--North Korean leader Kim Jong Un invited his South Korean counterpart for a summit in Pyongyang, presenting Seoul with a quandary: accept, and risk heightening differences with the U.S. over how to rein in the North's nuclear program, or decline, and risk a chance to build on a thaw in cross-border relations.

Moon Jae-in must now walk a fine diplomatic line as he tries to encourage North Koreainfo-icon's new interest in dialogue without straying too far from the "maximum pressure" approach championed by the United Statesinfo-icon, South Koreainfo-icon's military ally. 

Moon tried to keep both Pyongyang and Washington happy Saturday when he received the invitation, delivered by Kim Yo Jong, the North Korean leader's younger sister and close aide, during a meeting at the presidential Blue House in Seoul.

Moon responded in a noncommittal way, saying that he wanted to "create the environmentinfo-icon for that to be able to happen." But he also encouraged North Korea to "actively pursue" dialogue with the United States.

In a stark illustration of his efforts to pull in Pyongyang without pushing away Washington, Moon watched short-track speedskating with Vice President Pence at the PyeongChang Winter Olympicsinfo-icon on Saturday night. 

Then, after Pence had departed for home, Moon went to the women's ice hockey match with the North Korean delegation, including Kim Yo Jong. The two Koreas had combined teams and played simply as "Korea," wearing uniforms emblazoned with blue-and-white flags showing an undivided peninsula.

Pence had arrived in South Korea for the opening of the Olympics with a clear intention to isolate North Korea. He repeatedly called Kim Jong Un's regime "the most tyrannical" on Earthinfo-icon and studiously avoided talking to or even acknowledging the North Korean delegation, even when they sat just feet apart in the VIP box at the Opening Ceremonies. 

Shortly after leaving PyeongChang, Pence told reporters on Air Force Two that he had set out on the trip "to express American resolve regarding North Korea." 

"I was encouraged by the affirmation of our alliance and our common purpose, from both [Japanese] Prime Minister Abe and President Moon," Pence said. "I leave this trip encouraged that we will continue to work very closely to continue and intensify the maximum-pressure campaign that is underway against the regime in Pyongyang." 

Kim Yo Jong, sister of North Korean leader Kim Jong Un, will be a part of a top delegation attending the Winter Games opening ceremony in PyeongChang Feb. 9. (Reuters)

Asked later whether Moon's desire to accept the North Korean invitation to visit Pyongyang undermined Pence's trip or detracted from his message, a senior administration official said: "No, I don't think it does in the least."

But Kim Jong Un seems determined to divide the allies.

South Korea and the United States had agreed to postpone their annual spring military exercises until after the Olympics, partly to encourage North Korea to participate -- and to not act as a spoiler.

North Korea is calling for the exercises, which involve rehearsing attacks on the regime, to be canceled outright.

"Sending his sister with an invite for Moon to Pyongyang is well played by Kim Jong Un," said James Schoff, a Korea expert at the Carnegie Endowment for International Peace in Washington. "It will be hard for the allies to resume their military exercises during summit planning."

For that reason, Schoff said, Washington should work closely with Seoul to shape any North-South summit agenda, "defending the role of exercises for stability and insisting that denuclearization is front and center of any peace talks."

Pence wasn't trying to avoid the North Koreans at the Olympic ceremony, but he was trying to ignore them, the senior administration official said, adding that the North Koreans weren't the reason Pence was there. 

Some South Korean mediainfo-icon criticized Pence's decisions to attend the event for only five minutes, instead of sitting at the same table as the North Koreans, and to not stand when the joint Korean team entered the stadium. The left-wing Kyunghyang newspaper called the actions "deeply disrespectful."

North Korea's state media also sharply criticized Pence.

"Pence must know that his frantic acts of abusing the sacred Olympics for confrontational ruckus are as foolish and stupid an act as sweeping the sea with a broom," the North's state-run Korean Central News Agency said in a commentary published Saturday.

But the vice president's team saw it differently. Communications Director Jarrod Agen tweeted a laudatory review of Pence's evening: "VP stands and cheers for U.S. athletes. VP hangs out with U.S. athletes instead of dining with Kim regime. VP does not applaud N. Korea or exchange pleasantries w/ the most oppressive regime on earth," he wrote.

There were South Koreans who agreed with Pence's stance. 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.

Regardless of the different perspectives, Saturday was a remarkable day historically.

Kim Yo Jong, who is thought to be about 30 years old, is the first member of the North's ruling family to visit the South since the Korean Warinfo-icon began in 1950.

The catalyst for her visit was the Winter Olympics, which Moon is promoting as the "peace games." A total of about 500 North Koreans, including 22 athletes and 140 musicians, are attending.

In addition to Kim Yo Jong, the North Korean delegation included Choe Hwi, chairman of the National Sportsinfo-icon Guidance Committee; both are under U.S. sanctions for human rightsinfo-icon abuses for their roles in censoring information. Choe is also under international sanctions; South Korea had to seek a special exemption from the United Nationsinfo-icon for his travelinfo-icon.

The leader of the delegation was Kim Yong Nam, the 90-year-old who is technically North Korea's head of state and who was by Kim Jong Un's side as he presided over a huge military parade in Pyongyang the day before traveling to South Korea.

They all wore red pins over their hearts showing the first- and second-generation leaders of North Korea: Kim Yo Jong and Kim Jong Un's father and grandfather.

After their three hours of meetings and lunch on Saturday, Moon's spokesman said that Kim Yo Jong had come as her brother's "special envoy" and that the talks had taken place in a "friendly atmosphere."

Kim Yo Jong delivered a written letter and a verbal message from her brother inviting the South Korean president to Pyongyang at his earliest convenience. In a message in a guest book, she wrote: "I hope Pyongyang and Seoul will become closer in the hearts of Koreans and will bring unification and prosperity in the near future."

Twice, South Korean presidents, both of them progressives in whose steps Moon follows, have visited Pyongyang for inter-Korean summits, in 2000 and 2007. But both of those meetings were with Kim Jong Un's father. 

The third-generation leader, who took control of the state at the end of 2011, has shown little interest in engaging with the outside worldinfo-icon. He has not even been to Chinainfo-icon, North Korea's traditional ally, as leader and has snubbed recent high-ranking visitors.

Many analysts are skeptical about Kim Jong Un's motivations for this sudden outreach, especially as he continues to assert his desire to press ahead with his nuclear weaponsinfo-icon program to fend off the "hostile" United States.

Previous rounds of denuclearization talks, even when they ended in a deal, have soon collapsed when the Kim regime has breached the terms.

The sanctions imposed against the Kim regime after last year's intercontinental ballistic missileinfo-icon launches and huge nuclear test are now beginning to take effect, experts say, postulating that Kim Jong Un is now looking for the "weakest link" in the chain to try to break the sanctions.

In that regard, Kim Jong Un has scored a victory this week. Not only did he get South Korea to breach its own direct sanctions by allowing a North Korean ferry to bring musicians to the South for the Olympics, but he also persuaded the United Nations to suspend international sanctions, even if only for three days.