After arriving in Seoul, President Obama said Friday that “the United States and South Korea stand shoulder to shoulder...in our refusal to accept a nuclear North Korea.” (Reuters)

After arriving in Seoul, President Obama said Friday that “the United States and South Korea stand shoulder to shoulder...in our refusal to accept a nuclear North Korea.” (Reuters)

With North Korea making potential preparations for another underground nuclear test, President Obama said Friday that he saw no “magic bullet” to influence an already isolated nation whose advancing weapons program poses a “direct threat” to the United States.

A new nuclear test would provide the North with a key measuring post as it tries to create a reliable, miniaturized atomic weapon — one small enough to mount on a long-range missile that could strike the United States. The detonation would also highlight the dilemma facing the Obama administration, which has devoted relatively little political capital to addressing Pyongyang’s weapons program after years of failed diplomatic gambits.

“North Korea is already the most isolated country in the world — by far,” Obama said in a joint news conference with South Korean President Park Geun-hye at the presidential palace in Seoul. “Its people suffer terribly because of the decisions its leaders have made. And we are not going to find a magic bullet that solves this problem overnight.”

Obama’s arrival in Seoul coincided with a flourish of activity at North Korea’s mountainous nuclear test site, potential preparations for a fourth underground blast. Officials in Seoul cautioned that the apparent work — picked up by commercial satellites — could be a bluff or an attempt to stoke anxiety in the region.

Park said Friday that the North is “fully ready” to carry out the test technologically and could do so whenever it makes the political decision. Another test would “fundamentally change” the security situation in the region, she said without elaborating. Park also speculated that other Asian countries could join in a “nuclear arms race” as North Korea’s capabilities expand.

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Under leader Kim Jong Un, the North has vowed to never relinquish its weapons, even altering its constitution to say it is a “nuclear­-armed state.”

Analysts say the United States has based its “strategic patience” policy on the hope that North Korea, facing isolation and sanctions, would rethink its combativeness. Instead, Pyongyang has managed key advancements in its weapons program despite sanctions designed to cut off funding for such development. It has repeatedly tested ballistic missiles and atomic weapons in the face of international warnings and condemnation.

Washington and Pyongyang have gone more than two years without dialogue, and the six-

party talks — a multi-nation process designed to coax Pyongyang’s denuclearization — have been dormant since 2008. The Pentagon said this year in a report to Congress that Pyongyang’s weapons tests were a way of “gaining international recognition and de facto acceptance as a nuclear state.”

“The door has essentially been left open to [North Korea] by the ineffectiveness of previous diplomatic efforts,” said Evans Revere, a senior fellow at the Brookings Institution and a former diplomat who negotiated often with the North. “And they’re taking full advantage of that.”

Obama said Friday that the United States and its allies could respond to additional provocations by imposing sanctions that have “even more bite” and by highlighting the North’s horrific rights violations. Park and Obama also announced Friday that they were considering delaying a plan that would give Seoul control of its own troops during a war on the Korean Peninsula, rather than place them under U.S. command.

The transfer had been planned for December 2015. But the handoff has drawn criticism from some conservative South Koreans, who say deterrence of the North could falter if the militarily superior United States cedes a degree of control. Obama said Friday that the United States and South Korea also planned to enhance the “inter­-operability” of their missile defense systems.

The North’s primary goal, officials in Seoul and Washington say, is to construct a reliable nuclear-tipped missile. This requires both a miniaturized atomic weapon and an intercontinental ballistic missile capable of reaching orbit and reentering the atmosphere. Although intelligence officials have mixed assessments about Pyongyang’s capabilities, even the most generous assessment concedes that the nuclear missile would have a low reliability.

North Korea has only once placed a long-range missile into orbit, after numerous high-profile failures. North Korea said last year that it had successfully manufactured a smaller — or miniaturized — warhead, but there has been no way to verify that assertion. Previous blasts were relatively modest, several times smaller than the devices the United States dropped on Hiroshima and Nagasaki in Japan to end World War II.

If the North does test another weapon, it could lend clues about the nature of its arsenal. The most pressing question is whether the bomb uses plutonium or highly enriched uranium (HEU), both of which the North has pursued in its bid to produce fissile material. North Korea has a small stockpile of plutonium, which it used for its first two tests in 2006 and 2009.

But the North’s uranium production is a relatively newer program — and more challenging to stop. Weapons experts suspect that the North has a clandestine network of underground uranium-enrichment plants. There is no estimate of how much, if any, weapons-grade uranium the North has on hand. After last year’s atomic test, a network of sensors detected what are known as noble gasses but could not determine whether the blast used plutonium or uranium.

“The plutonium [stockpile] we know pretty well,” said Jeffrey Lewis, an East Asia nonproliferation expert at the Monterey Institute of International Studies. “The real issue is, we have no idea how much HEU is out there. If there’s an underground HEU factory churning stuff out, we just don’t know about it.”

The 38 North Web site, run by a U.S. think tank, said Friday that new commercial satellite images showed increased activity and vehicles near the entrances of two purported test tunnels — steps consistent with preparations before a February 2013 blast. Satellite imagery also showed newly arrived communications vehicles, according to the site, which is a project of the U.S.-Korea Institute at Johns Hopkins University.

Before last year’s test, preparations peaked two or three days before the detonation; personnel, vehicles and equipment were withdrawn just before the blast.

“Whether North Korea will follow the same timeline in 2014 remains unclear,” the think tank said.