The US vice-president has arrived at the gateway to the demilitarised zone (DMZ), the area diving the two Koreas.
Mike Pence, who is set to meet the acting president of South Korea later, will visit four nations on his 10-day Asia tour.
At the weekend, North Korea’s latest missile test failed, exploding shortly after take off.
Tensions between North Korea and the US have escalated in recent weeks, with heated rhetoric from both sides.
On Monday the US and South Korea launched a joint air force military exercise to ensure readiness against North Korea, according to South Korean media.
Mr Pence took a helicopter to Camp Bonifas, a United Nations military compound near the DMZ.
He is expected to visit the truce village of Panmunjom, where the Korean War armistice was signed, which is located within the DMZ.
Mr Pence, whose father had fought in the Korean War, told reporters that the alliance between the US and South Korean militaries was “historic” and “a testament to the unshakeable bond between our people”.
The US vice president arrived in Seoul on Sunday, hours after North Korea fired a ballistic missile, which he called a “provocation”.
Lt Gen HR McMaster, the US top security adviser, has said they are working on a “range of options” with China on North Korea, the first confirmation the two countries are working together on how to deal with the North Korean issue.
Meanwhile about 1,000 US airmen and fighter jets are taking part in a combat training exercise in South Korea, reported Yonhap news agency. South Korea has sent about 500 personnel and planes. The Max Thunder exercise will last for two weeks.
Besides Sunday’s failed missile launch, North Korea has held a series of large-scale events in the past week including a massive celebration and military parade on Saturday.
It has denounced the US deployment of an aircraft carrier group to the region, saying it would respond by “force of arms” to “reckless moves”.
China, Pyongyang’s sole major ally, has reiterated its call for North Korea to stop all tests, and has also called for a peaceful solution.
US President Donald Trump, who stated last week that the US and its allies may “deal with” Pyongyang if China did not, said on Sunday that Beijing was “working with us on the North Korean problem”.