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PANMUNJOM, Korea — On his first visit to the tense but eerily quiet frontier between North and South Korea as U.S. secretary of defense, Jim Mattis conveyed the message he hopes will win the day: Diplomacy is the answer to ending the nuclear crisis with the North, not war.

He made the point over and over — at the Panmunjom “truce village” where North literally meets South; at a military observation post inside the Demilitarized Zone, and in off-the cuff comments to U.S. and South Korean troops.

“We’re doing everything we can to solve this diplomatically — everything we can,” he told the troops after alighting from a Black Hawk helicopter that had ferried him to and from the border some 25 miles north of central Seoul.

“Ultimately, our diplomats have to be backed up by strong soldiers and sailors, airmen and Marines,” he added, “so they speak from a position of strength, of combined strength, of alliance strength, shoulder to shoulder.”

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At Panmunjom, where the armistice ending fighting between North and South was signed in July 1953, Mattis quoted Secretary of State Rex Tillerson as saying, “Our goal is not war.” The aim, he said, is to compel the North to completely and irreversibly eliminate a nuclear weapons program that has accelerated since President Donald Trump took office.

The neighbors are technically still in conflict because the Korean War ended with the armistice, rather than a peace treaty. In the decades since, the North has issued regular apocalyptic threats, saying it will engulf Seoul in "a sea of fire."

Related: Breakdown in North Korea Talks Sounds Alarms on Capitol Hill

Despite unanimous condemnation by the U.N. Security Council of the North’s missile launches and nuclear tests, “provocations continue,” Mattis said.

Mattis called the North “an oppressive regime that shackles its people, denying their freedom, their welfare and their human dignity in pursuit of nuclear weapons and their means of delivery in order to threaten others with catastrophe.”

He noted that earlier this week in the Philippines, he and South Korean Defense Minister Song Young-moo joined Southeast Asian defense ministers in committing to a diplomatic solution to the North Korea problem, even though Pyongyang and its young leader, Kim Jong Un, show no interest in negotiations.

North Korean soldiers, left, look at the South side while U.S. Defense Secretary Jim Mattis visits the truce village of Panmunjom in the Demilitarized Zone on Friday. Jung Yeon-je / AP

Two other developments Thursday showed the U.S. intention to continue building diplomatic and economic pressure on Pyongyang. The Trump administration imposed sanctions on 10 North Korean officials and organizations over human rights abuses and censorship, including a diplomat in China accused of forcing North Korean asylum seekers home.

Meanwhile, a rare military exercise involving three of the U.S. Navy’s aircraft carrier strike groups was being planned for next month in the Asia Pacific, a U.S. official said. The likely exercise would happen around the time that Trump travels to the region, including to Seoul.

Trump entered office declaring his commitment to solving the North Korea problem, asserting that he would succeed where his predecessors had failed. His administration has sought to increase pressure on Pyongyang through U.N. Security Council sanctions and other diplomatic efforts, but the North hasn’t budged from its goal of building a full-fledged nuclear arsenal, including missiles capable of striking the U.S. mainland.

On Saturday, Mattis will be joined by Gen. Joseph Dunford, chairman of the Joint Chiefs of Staff, in annual consultations with South Korean defense officials. They are expected to admonish North Korea, vow to strengthen allied defenses, and discuss prospects for eventually giving South Korea wartime control of its own forces.