US Acknowledges ‘Seismic Activity’ Near North Korean Nuclear Site

After North Korea said it successfully tested a hydrogen bomb, the U.S. late Tuesday acknowledged "seismic activity" near a nuclear site in the country but stopped short of confirming the claim.

"We are aware of seismic activity on the Korean Peninsula in the vicinity of a known North Korean nuclear test site and have seen Pyongyang's claims of a nuclear test," John Kirby, State Department spokesman, said in a statement.

"We are monitoring and continuing to assess the situation in close coordination with our regional partners," he added. "While we cannot confirm these claims at this time, we condemn any violation of UN Security Council Resolutions and again call on North Korea to abide by its international obligations and commitments."

The Defense Department didn't comment on the news reports but shared the State Department statement.

North Korea said its test of a "miniaturized" hydrogen bomb was a "perfect success" -- a claim that, if true, would confirm fears among Western observers that it was closer to developing nuclear warheads for long-range missiles capable of reaching the U.S.

The test would mark the country's third demonstration of nuclear technology since 2006 and its first thermonuclear detonation. A hydrogen bomb is far more powerful than an atomic bomb and relies on the heat generated from nuclear fission to trigger a fusion reaction.

North Korea has steadily increased the explosive power, or yield, of its previous nuclear tests, from less than a kiloton in 2006 to about two kilotons in 2009 to six to seven kilotons in 2013, according to the Congressional Research Service. The yield of the latest test wasn't immediately known.

The first two tests are believed to have used plutonium rather than highly enriched uranium. Obtaining the fissile material is the main hurdle in building a nuclear weapon. North Korea is estimated to have between 30 and 50 kilograms of separated plutonium -- enough for four to seven weapons, according to the 2013 report from the CRS.

North Korea has spent decades trying to perfect a multistage, long-range missile to eventually carry smaller versions of nuclear bombs, the Associated Press reported. After several failures, it put its first satellite into space with a long-range rocket launched in December 2012, the AP reported.

After North Korea's 2013 underground nuclear detonation, the U.S. flew pairs of stealth aircraft over South Korea, including F-22 fighter jets and nuclear-capable B-2 bombers, in a show of force against the North.

"We have consistently made clear that we will not accept it as a nuclear state," Kirby said. "We will continue to protect and defend our allies in the region, including the Republic of Korea, and will respond appropriately to any and all North Korean provocations."

--Brendan McGarry can be reached at brendan.mcgarry@military.com. Follow him on Twitter at @Brendan_McGarry.