The United States vowed to defend itself and its regional allies Tuesday after North Korea again stepped up its warlike rhetoric and the UN warned that the crisis could spin out of control.
Standing side-by-side with counterpart South Korean Foreign Minister Yun Byung-se, US Secretary of State John Kerry denounced "an extraordinary amount of unacceptable rhetoric from the North Korean government in the last days.
"Let me be perfectly clear here today. The United States will defend and protect ourselves and our treaty ally, the Republic of Korea," Kerry said, also vowing to stand by Japan as the latest crisis with Pyongyang unfolds.
Kerry was speaking after the North triggered renewed alarm by warning it would reopen the Yongbyon reactor -- its source of weapons-grade plutonium -- in the latest in a series of increasingly bellicose threats.
He said the recent posturing by new North Korean leader Kim Jong-Un was provocative as well as "dangerous and reckless."
Yun, who is on his first visit to Washington since becoming foreign minister in the new government of President Park Geun-Hye, renewed Seoul's commitment to working with Washington.
"We agree to further strengthen credible and robust deterrence vis-a-vis North Korea's nuclear and conventional provocations," Yun said.
But UN chief Ban Ki-moon warned the crisis could spiral out of control, stressing that "nuclear threats are not a game."
"The current crisis has already gone too far ... Things must begin to calm down," the former South Korean foreign minister told a press conference in Andorra, adding that negotiations were the only viable way forward.
Tensions have been escalating on the Korean peninsula since the North held a nuclear test in February, having launched a long-range rocket in December.
Last week in a rare show of force in the region, Washington deployed nuclear-capable US B-52s, B-2 stealth bombers and a US destroyer to South Korean air and sea space.
It has also called on China and Russia to do more to rein in North Korea, after Beijing earlier voiced regret over Pyongyang's announcement.
"It is not a mystery to anyone that China has influence with North Korea," White House spokesman Jay Carney told reporters.
"We have in the past and are now urging China to use that influence to try to affect North Korean behavior. That is also true of our (conversations) with the Russians."
A Pyongyang government nuclear energy spokesman said the plans for Yongbyon would involve "readjusting and restarting" all facilities at the complex, including a uranium enrichment plant and the five-megawatt reactor.
The North shut down the Yongbyon reactor in July 2007 under a six-nation aid-for-disarmament accord, and destroyed its cooling tower a year later.
Experts say it would take six months to get the reactor back up and running, after which it would be able to produce one bomb's worth of weapons-grade plutonium a year.
North Korea revealed it was enriching uranium at Yongbyon in 2010 when it allowed foreign experts to visit the centrifuge facility there, but insisted it was low-level enrichment for energy purposes.
Kim Yong-Hyun, a North Korea expert at Seoul's Dongguk University, said Tuesday's nuclear initiative was different from the military bluster of recent weeks.
"This goes beyond mere provocation. It's a strong, tangible move and perhaps the one that will force the US into the direct dialogue Pyongyang wants," Kim said.
The North has substantial uranium ore deposits which provide a quick route to boosting reserves of fissile material, while plutonium has the advantage of being easier to miniaturise into a deliverable nuclear warhead.
Many observers believe the North has been producing highly-enriched uranium in secret facilities for years, and that the third nuclear test it conducted in February may have been of a uranium bomb.
Its previous tests in 2006 and 2009 were both of plutonium devices.