US Vice President Kamala Harris toured South Korea's heavily fortified border with the nuclear-armed North on Thursday, part of a trip aimed at strengthening the security alliance with Seoul.

Pyongyang conducted two ballistic missile launches in the days before Harris' arrival, continuing a record-breaking streak of weapons tests this year.

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.

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.

Speaking at the Demilitarized Zone (DMZ), 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.

South Korean and US officials have warned for months that Kim Jong Un is preparing to conduct another nuclear test.

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. (AFP)