POTCHEON,South Korea - US Defence Secretary Chuck Hagel toured the South-North Korean border on Monday as he kicked off a trip to key regional allies in the battle to halt Pyongyang's nuclear weapons programme.
"There is no margin of error up here," Hagel told reporters at the heavily-fortified Demilitarized Zone (DMZ) that separates the two Koreas.
Hagel's visit to South Korea and Japan 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.
His third trip to Asia as Pentagon chief is expected to underline the importance of the US military alliances with Seoul and Tokyo, not just in the context of the North Korea threat but also China's growing strategic power.
On Monday, Hagel watched a live-fire exercise at a military complex 10 kilometres (six miles) south of the border with the North.
He then 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 agreed to take over wartime operational command of all troops starting in 2015, a decision that was already delayed from a 2012 target date.
But South Korean defence policymakers now say they need more time to prepare for the transition, citing increased military threats from the North after its February nuclear test.