US Defense 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.

The issue is a divisive one in South Korea.

Left-leaning opposition parties view the existing command structure as quasi-colonial and tend to favour a rapid transition.

Some right-wing hardliners also perceive a slight to national pride, but the conservative establishment also believes US command is the best guarantee of long-term US commitment to South Korea's defence.

US officials have reaffirmed the 2015 schedule and, in a briefing to reporters on his flight to Seoul, Hagel noted that the South Korean military had become "much more sophisticated, much more capable" over the past 10 years.

But he added it would be premature "to make any final decision" on the issue, which he will also discuss during a meeting with President Park Geun-Hye.

Tensions on the Korean peninsula, which sky-rocketed after the nuclear test, have eased a little in the past month, but concerns over Pyongyang's nuclear ambitions remain acute.

Analysis of recent satellite images suggest it has restarted the plutonium reactor that provided the fissile material for at least two of its three nuclear tests, and may have doubled its uranium enrichment capacity at the Yongbyon nuclear complex.

Hagel said the recent use of chemical weapons in Syria and the international community's response had been followed with particular concern by officials in Seoul.

"The South Koreans are very concerned because the North Koreans possess a very significant stockpile of chemical weapons," he said.

North Korea and its main ally China have both urged a resumption of six-party talks on the North's nuclear programme, but Washington and Seoul insist Pyongyang must demonstrate some tangible commitment to denuclearisation before any substantive dialogue can be held.

The six-party process, which the North exited in 2009, involves the two Koreas, China, Japan, Russia and the United States.

During his stay in Seoul, Hagel will attend a military parade on Tuesday to mark the 65th anniversary of the founding of South Korea's armed forces.

On display will be a cruise missile that the South says is capable of surgical strikes on the North Korean leadership.

On Wednesday, he leaves for Japan, which is increasingly concerned by what it considers China's dangerous behaviour over a disputed island chain in the East China Sea.

Prime Minister Shinzo Abe has cited the territorial row in his calls for beefing up the Japanese military.