North Korea announced plans on Tuesday to restart a mothballed nuclear reactor that has been closed since 2007, but emphasized it was seeking a deterrent capacity, rather than repeating recent threats to attack South Korea and the United States.
The state-owned KCNA news agency said North Korea would restart all nuclear facilities for both electricity and military uses.
The announcement came amid soaring tensions on the Korean Peninsula as the United States bolstered its forces in the region after a series of threats by Pyongyang to attack U.S. bases in the Pacific and to invade South Korea.
North Korea, one of the most isolated and unpredictable states in the world, conducted its third nuclear test in February but is believed to be some years away from developing nuclear weapons, although it claims to have a deterrent.
A speech by the North's young leader Kim Jong Un, delivered on Sunday but published in full by KCNA on Tuesday, appeared to dial down the prospects of a direct confrontation with the United States as he stressed that nuclear weapons would ensure the country's safety as a deterrent.
"Our nuclear strength is a reliable war deterrent and a guarantee to protect our sovereignty," Kim said. "It is on the basis of a strong nuclear strength that peace and prosperity can exist and so can the happiness of people's lives."
Kim's speech, delivered to the central committee meeting of the ruling Workers Party of Korea, appeared to signal a small shift from threats against South Korea and the United States, but it was some distance from any kind of end to the crisis.
"The fact that this (speech) was made at the party central committee meeting, which is the highest policy-setting organ, indicates an attempt to highlight economic problems and shift the focus from security to the economy," said Yang Moo-jin of the University of North Korean Studies in Seoul.
But if Pyongyang follows through with its plan to restart the nuclear facilities, it will have longer-term security implications for the region.
Reactivating the aged Soviet-era reactor at the Yongbyon nuclear plant will produce plutonium, a tested path to acquire more fissile material than a uranium enrichment program.
It was unclear how quickly the Yongbyon plant, whose cooling tower was destroyed as part of a de-nuclearization deal, would take to restart and it was impossible to verify whether it was still connected to North Korea's antiquated electricity grid at all.
"It was a reactor that was nearing obsolescence with a cooling tower that wasn't functioning properly when it was blown up. It could mean they've been rebuilding quite a few things," said Yoo Ho-yeol, North Korea specialist at Korea University in Seoul.
The move to restart the reactor comes as a big blow to China's stated aim of restarting de-nuclearization talks on the Korean peninsula, prompting a foreign ministry spokesman in Beijing to express regret at the decision.
As well as restarting the 5MW reactor at Yongbyon, the North's only known source of plutonium for its nuclear weapons program, KCNA said a uranium enrichment plant would also be put back into operation.
The nuclear plant's output would be used to solve what KCNA termed an "acute shortage of electricity" and to bolster "the nuclear armed force".
After being hit with U.S. sanctions for conducting the February nuclear test and what it has viewed as "hostile" military drills being staged by Seoul and Washington in the South, Pyongyang had threatened a nuclear strike on the United States, missile strikes on its Pacific bases and war with South Korea.
Washington, which has said it has not seen any evidence of hostile North Korean troop moves, deployed a warship off the Korean coast overnight.
The United States earlier bolstered forces staging joint drills with South Korea with Stealth fighters and has made bomber overflights in a rare show of strength.
Much of the rhetoric that has come from Pyongyang in recent weeks has been a repeat of previous bouts of anger, but the length and intensity has been new, leading to concerns that the tensions could spiral into clashes.
In Washington, the White House has said the United States takes North Korea's war threats seriously. But White House spokesman Jay Carney said on Monday: "I would note that despite the harsh rhetoric we are hearing from Pyongyang, we are not seeing changes to the North Korean military posture, such as large-scale mobilizations and positioning of forces."
A U.S. defense official said on Monday the USS McCain, an Aegis-class guided-missile destroyer used for ballistic missile defense, was positioned off the peninsula's southwestern coast.
It was not immediately clear where the ship was on Tuesday.
In Pyongyang, the party congress meeting and a subsequent assembly of the country's rubber-stamp parliament reiterated the usual anti-American rhetoric and criticized South Korea, but the mood appeared to have changed.
The pariah state has once again started emphasizing economic development as it shifts to the major April 15 celebration of the birth of its founder, Kim Il Sung, the grandfather of the current ruler.
For the young Kim, it appears that cementing control of the party and state had now taken top priority as well as improving living standards in a country whose economy is smaller than it was 20 years ago, according to external assessments.
Kim appointed a handful of personal confidants to the party's politburo, further consolidating his grip on power in the second full year of his reign.
Former premier Pak Pong-ju, a key ally of the leadership dynasty, was re-appointed to the post from which he was fired in 2007 for failing to implement economic reforms.
Pak, believed to be in his 70s, is viewed as a key confidant of Jang Song-thaek, the young Kim's uncle and also a protege of Kim's aunt. Pak is viewed as a pawn in a power game that has seen Jang and his wife re-assert power over military leaders.
Additional reporting by Paul Eckert, Phil Stewart, David Alexander and Jeff Mason in Washington; Editing by David Chance and Raju Gopalakrishnan
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