The US Secretary of State, Hillary Clinton, and Defence Secretary Robert Gates have visited the Demilitarized Zone separating North and South Korea.
Mr Gates said they wanted to show solidarity with their allies in Seoul.
The visit comes ahead of a joint US-South Korea military exercise due to start later this week, intended to send a message of deterrence to North Korea.
Pyongyang has been accused of sinking a South Korean warship with a torpedo in March, but it denies any involvement.
On Tuesday, the nominee to be US director of national intelligence warned that incident might herald a "dangerous new period".
James Clapper told a Senate hearing that Pyongyang might seek "to advance its internal and external political goals through direct attacks".
'Sabre rattling'
The BBC's John Sudworth in Seoul says the appearance of two senior members of the US government in the heavily-fortified frontier on Wednesday appeared to take the North Korean soldiers stationed just a few metres away by surprise.
One of them ran down the steps towards the conference huts that straddle the border as Mrs Clinton and Mr Gates were given a tour of the South Korean side, our correspondent says.
"Although it may be a thin line, these two places are worlds apart," Mrs Clinton told reporters, praising the South's "extraordinary progress" since the Korean War.
"By contrast, the North has not only stagnated in isolation, but the people of the North have suffered for so many years," she added.
Mrs Clinton said she wanted Pyongyang to know "there is another way".
"But until they change direction, the United States stands firmly on behalf of the people and government of the Republic of Korea, where we provide a stalwart defence along with our allies and partners."
Mr Gates said their visit to South Korea was meant as a message of solidarity with an ally, and a message of deterrence towards the North.
North and South Korea technically remain at war because their three-year conflict ended in an armistice in 1953 and no peace treaty was signed. The US has since stationed thousands of troops in South Korea.
Mrs Clinton and Mr Gates will later hold talks with their South Korean counterparts and senior military commanders, during which they will discuss the exercises being held in response to the attack in the Yellow Sea on the corvette, Cheonan, which left 46 sailors dead.
The first manoeuvres, in the Sea of Japan (East Sea) for four days from Sunday, will involve the nuclear-powered aircraft carrier USS George Washington and 20 other ships and submarines, as well as 100 aircraft and 8,000 personnel. Later exercises will take place in the Yellow Sea.
China has said it objects to any foreign military operations in the Yellow Sea, which is on the western side of the Korean Peninsula. North Korea has dismissed the exercises as "sabre rattling".
