On his first visit to the tense but eerily quiet frontier between North and South Korea as Mattis conveyed the message he hopes will win the day: Diplomacy is the answer to ending the nuclear crisis with the North, not war.
It has backed USA -led United Nations sanctions on North Korea following a series of prohibited arms tests, some of which coincided with this year's political events deemed highly important for Xi.
"Any use of nuclear weapons will be met with a massive military response".
The CIA has said North Korea could be only months away from developing the ability to hit the United States with nuclear weapons, a scenario Trump has vowed to prevent.
The tension in the border is building up as North Korean Foreign Minister threatened last month while visiting NY for the UN General Assembly that his country will not hesitate using hydrogen bomb, hours after US President Donald Trump threatened to "totally destroy" North Korea in a speech to the UN.
"North Korea has accelerated the threat that it poses to its neighbors and the world through its illegal and unnecessary missile and nuclear weapons programs", he added, emphasizing that the USA "does not accept a nuclear North Korea".
South Korean officials said the 10 man crew were released on Friday evening.
The sanctioned entities include the Workers' Party of Korea.
US intelligence experts say Pyongyang believes it needs the nuclear weapons to ensure its survival and have been skeptical about diplomatic efforts, focusing on sanctions, to get Pyongyang to denuclearize.
Mattis was briefed by US and South Korean Joint Security Area Commanders Lt. Col. Matthew Farmer and Lt. Col. Kwon Young-hwan at the Observation Post Ouellette.
One issue they said was not discussed during their meetings was whether the United States would once again deploy tactical nuclear weapons to South Korea, a practice that was stopped at the end of the Cold War.
Still, any attempt to force the North to denuclearize could have devastating consequences, thanks in part to the large amount of artillery trained on Seoul.
Defense Minister Song suggested that USA and South Korean missile defenses simply could not stop all of the weapons trained on the South.
He was in the South Korea capital for annual defence talks between the two allies.