SEOUL (AFP) - South Korea and the United States signed a strategic pact on Wednesday to prevent a North Korean nuclear attack and agreed to review a planned switch of US wartime command over South Korean troops.
The pact establishes a "strategic framework" for dealing with "key North Korean nuclear threat scenarios" that have escalated since the North's third nuclear test in February.
It was signed by visiting US Defence Secretary Chuck Hagel, who reiterated US commitment to using all its military capabilities - conventional and nuclear - to provide South Korea with an "extended deterrence that is credible, capable and enduring".
Although neither side provided any specific details of the measures envisaged by the new strategy, South Korean Defence Minister Kim Kwan Jin said it would "greatly enhance the efficacy" of the alliance's existing deterrence capacity.
Observers said the plan, which is certain to be condemned by North Korea, was largely a confidence-building measure to signal the depth of Washington's support for South Korea against any provocation from Pyongyang.
South Korea is protected by the US nuclear umbrella and there are currently nearly 30,000 US troops stationed in the country.
But Seoul insists that the alliance must respond to what Mr Kim described as the "vastly different" security situation on the Korean peninsula following the North's nuclear test.
To that end, it has requested an extension of US wartime command over South Korean troops - scheduled to end in 2015.
In case of war with North Korea, the alliance currently calls for the US military commander to lead the US troops deployed to the country, as well as South Korea's 640,000-strong force.
Seoul argues that the transition to South Korean command should be postponed until the nuclear threat from Pyongyang has been neutralised.
Washington has indicated it wants to keep to the original schedule, but Mr Hagel said he had listened "very seriously" to Seoul's concerns and promised further consultations on the issue.
"We are very optimistic we will have an agreement," he added.
At a joint press briefing with Mr Kim, Mr Hagel also stressed that the new deterrent strategy covered all the North's weapons of mass destruction, including chemical weapons.
According to South Korean defence officials, North Korea has up to 5,000 tonnes of chemical arms - an alleged stockpile that has been highlighted by the use of such weapons in Syria.
"There should be no doubt that North Korean use of chemical weapons would be completely unacceptable," Mr Hagel said.
The North's nuclear test in February - its third and most powerful to date - triggered months of heightened military tensions on the Korean peninsula.
Pyongyang threatened pre-emptive nuclear strikes against South Korea and the United States, while the Pentagon responded by deploying nuclear-capable B2 stealth bombers during joint military exercises with Seoul.
And in March, Seoul and Washington signed another pact providing for a joint military response even to low-level provocation by North Korea.
Tensions have since eased, but acute concerns remain over the North's nuclear weapons programme after satellite image analysis suggested it had expanded its production of weapons-grade fissile material.
"North Korea has increased its threat clearly against South Korea, and against the United States. It has increased its capability," Mr Hagel said.
But he also noted that South Korea's military has grown "stronger, more professional and more capable" over the past decade.
On Tuesday, Mr Hagel had attended South Korea's largest military parade for a decade which showcased a new cruise missile capable of pin-point strikes anywhere in North Korea.
The parade was condemned by North Korea as "an unprecedented display of lunatic hostility" that was made in collusion with the United States and aimed at "spurring preparations to attack the North".