US Defense Secretary Leon Panetta pledged Wednesday to preserve a "nuclear umbrella" protecting close ally South Korea, a day after the US held talks with Seoul's hostile neighbour North Korea.

"I've come here because in many ways this is the frontline," Panetta told some 300 US servicemen at the giant Yongsan base in Seoul.

"Six decades later (after the 1950-1953 Korean War), the US remains fully committed to the security of South Korea", he said.

Some 28,500 US troops are stationed in the South. Cross-border tensions have been high for the past year, after the South accused the North of mounting two border attacks in 2010 that killed a total of 50 South Koreans.

The US withdrew atomic weapons from the South almost 20 years ago but guarantees to provide a nuclear deterrent to any nuclear attack on it.

Panetta, who is on the last leg of a tour which also took him to Indonesia and Japan, emphasised the US defence commitment despite a flurry of diplomacy designed to revive six-nation talks on the North's nuclear disarmament.

US and North Korean officials held talks Monday and Tuesday in Geneva to try to set terms for a resumption of the negotiations, their second such meeting in three months.

Chief US envoy Stephen Bosworth described the talks as "very positive" but cautioned that not all differences could be quickly overcome.

The North quit the six-party forum in April 2009, a month before staging its second atomic weapons test.

It has since repeatedly said it wants to return without preconditions to the negotiations grouping the two Koreas, the US, China, Russia and Japan.

Washington and its allies say it must first take action to show its sincerity, such as shutting down a uranium enrichment plant that could be converted to make nuclear weapons.

China, which has hosted the talks since 2003, sent its vice-premier Li Keqiang to both North and South Korea this week to try to restart them.

Li met the North's leader Kim Jong-Il in Pyongyang and Wednesday held talks with the South's President Lee Myung-Bak.

"I told Chairman Kim several times that it is important to realise denuclearisation of the Korean peninsula and protect peace and stability," Lee's office quoted the vice-premier as saying.

Lee told his guest that he hopes China -- which is the North's closest ally but the South's biggest trading partner -- "will continue to play an important role in denuclearising the Korean peninsula and leading North Korea to reform and openness".

Panetta, in an article in Wednesday's Chosun Ilbo newspaper, said the US and South Korean military "stand prepared to defeat the North should it ever force war upon us.

"It is important to send this signal because North Korea remains a serious threat. Pyongyang has demonstrated its willingness to conduct provocations that target innocent lives," he wrote.

Panetta said the US and South Korea are developing capabilities to address the North's ballistic missile threats, and strengthening operational planning.

In addition, the US "will ensure a strong and effective nuclear umbrella over the ROK (South Korea) so that Pyongyang never misjudges our will and capability to respond decisively to nuclear aggression".

The defense secretary during his three-day visit will stress the two countries' capability to deter provocations and to defeat the North if deterrence fails, a senior official travelling Panetta told reporters.

He would also share views with his counterparts on the Geneva meeting, the official said.

"Our experience is that our North Korean friends go through cycles of diplomatic engagement and provocation. We need to be prepared for how that cycle may play itself out in the next turn."