PANMUNJOM, South Korea - US Defence Secretary Chuck Hagel toured the South-North Korean border Monday, ahead of talks on switching command of combined US and South Korean forces in the event of war with the North.
"There is no margin of error up here," Hagel told reporters at the heavily fortified Demilitarized Zone (DMZ) that still separates the two Koreas 60 years after the end of the Korean War.
Hagel was on the first leg of a trip to South Korea and Japan - two key regional military allies with a major stake in the battle to halt Pyongyang's nuclear weapons programme.
His visit follows signs that North Korea may be expanding its weapons-grade fissile material output even while calling for the resumption of six-party denuclearisation talks.
After watching a live-fire exercise near the border, Hagel toured the DMZ - once described by former US president Bill Clinton as the "scariest place on earth" - with his South Korean counterpart Kim Kwan-Jin.
"This is probably the only place in the world where we have always a risk of confrontation, where two sides are looking clearly and directly at each other," he told reporters at the Panmunjom truce village where the Korean War armistice was signed.
Hagel and Kim are due to hold talks on Tuesday which are likely to focus on Seoul's request for an extension of US wartime command over South Korean troops.
In the event of war with North Korea, the alliance currently calls for the US military commander to lead the 28,500 US troops deployed to the country, as well as South Korea's 640,000-strong force.
South Korea had agreed to take over wartime operational command of all troops starting in 2015, a decision that was already delayed from a 2012 target date.
South Korean defence policymakers now say they need more time to prepare for the transition, citing increased military threats from the North after it held a nuclear test in February.