"I know that people are watching for a wedge between South Korea, Republic of Korea, in other words, and the United States", Mattis said.
North Korean state media on Monday said its high-level delegation's trip to South Korea was "meaningful" and it improved stalled inter-Korean relations.
South Korean President Moon Jae-in has been using the Games in his efforts to break the ice with the North and pave the way for talks over the North's weapons programme.
"The US and South Korea has maintained a consistent stance that dialogue between South and North Korea and the talks between the US and North Korea should follow one after another to induce the denuclearization of North Korea and that continued sanctions and pressure is important", the ministry said in a statement. "Do you want to make Pyeongchang Olympics a "lie Olympics"?" he asked.
But the previous agreements broke down as North Korea pressed ahead with its nuclear program.
That's the precondition for the USA talking with North Korea, and all the signals so far we've seen from Washington are very stubborn.
President Donald Trump tweeted his congratulations to South Korea for playing host a day before the Games began, but had been silent on the topic since.
Winter Olympic Medal Standings As Of Sunday Night
The Netherlands comes in second place in the total number of medals with five, including two gold, two silver, and one bronze. They finished behind the Olympic Athletes of Russian Federation and Canada, who took silver and gold, respectively.
BBC had reported that the coming of Kim Yo-Jong to South Korea is itself a major event, as no member of the ruling family from North Korea had visited South Korea after 1953 when the Korean war ended.
"Under a strong position for denuclearization of the Korean peninsula, Korea will faithfully implement the global sanctions on North Korea, while also adhering to the principle of resolution through peaceful means", the statement said. South Koreans are divided on Kim Yo Jong, the sister of the North's leader, and her landmark visit to their country.
North Korean leader Kim Jong Un has expressed a willingness to hold talks with South Korea, and athletes from each nation marched in the opening ceremony under a united flag.
Pence was at pains to insist that Washington and Seoul were on the same page as he returned to Washington from leading the United States delegation to the Olympics.
As the Trump administration's "maximum pressure" campaign heats up, it increasingly appears effective as fuel prices soar and businesses on both sides of the China-North Korea border fold.
The downside for Washington is that it could expose growing differences with Seoul on the best way to deal with North Korea and achieve the ultimate goal of denuclearization.
By "snubbing" the North Korean delegation, Pence made himself and his treatment of the North Koreans the story.