The US vice president says America's commitment to South Korea's defence is "ironclad", adding that the allies are "aligned" in their response to North's weapons programmes.
US Vice President Kamala Harris has toured South Korea's heavily fortified border with the nuclear-armed North, part of a trip aimed at strengthening the security alliance with Seoul.
Speaking at the Demilitarised Zone (DMZ) on Thursday, Harris said that US and South Korean soldiers were "serving shoulder to shoulder...to maintain the security and the stability of this region of the world".
She said the US commitment to South Korea's defence was "ironclad", adding that the allies were "aligned" in their response to the growing threat posed by the North's weapons programmes.
The allies both want "a complete denuclearisation of the Korean peninsula", but in the interim they are "ready to address any contingency", she said.
At an observation post atop a steep hill overlooking North Korea, Harris peered through bulky binoculars as US and South Korean soldiers pointed out features, including defences, in the area.
"It's so close," she said.
South Korean and US officials have warned for months that Kim Jong Un is preparing to conduct another nuclear test.
Harris decried North Korea's "brutal dictatorship, rampant human rights violations and an unlawful weapons program that threatens peace and stability".
Harris also visited the Panmunjom Truce Village — where then-US president Donald Trump met the North's Kim Jong Un in 2019 — and talked to US soldiers at Camp Bonifas in the Joint Security Area.
On the North Korean side of the border at Panmunjom, guards in hazmat suits could be seen watching as Harris was shown the demarcation line between the two countries — which remain technically at war.
Earlier Thursday, Harris met President Yoon Suk-yeol for talks dominated by security issues.
Harris, America's first woman vice president, also met what the White House called "groundbreaking women leaders" of South Korea to discuss gender equality issues, a topic she said she raised with Yoon during their talks.
Harris arrived in Seoul after a trip to Japan, where she attended the state funeral of assassinated former prime minister Shinzo Abe.
Harris' trip to the DMZ is likely to infuriate Pyongyang, which branded United States House Speaker Nancy Pelosi the "worst destroyer of international peace" when she visited the border in August.
Pyongyang conducted two banned ballistic missile launches in the days before Harris's arrival, continuing a record-breaking streak of weapons tests this year.
And on Wednesday, the South's spy agency said North Korea's next nuclear test could happen as soon as next month, likely after China's upcoming party congress but before the US midterms.
The isolated regime has tested nuclear weapons six times since 2006, most recently in 2017. Earlier this month it changed its laws, declaring itself an "irreversible" nuclear power.
Seoul announced on Thursday that it would hold trilateral anti-submarine drills with Japan and the US, the first such exercises since 2017.