Secretary of State John Kerry called North Korea's hostile threats "unacceptable," but said the communist country could come "back to the table" of the global community if they committed to a nuclear-free Korean peninsula.
Kerry addressed reporters in a dual press conference with his South Korean counterpart, Yun Byung-se, in Washington after the two discussed the escalating tensions and near daily warlike threats.
Kerry and Yun's meeting came shortly after North Korea vowed to restart a nuclear reactor that can make on bomb's worth of plutonium a year, a move the State Department said would be "extremely alarming."
"They have an option, and that option is to enter into negotiations and begin to focus on the needs of their people," Kerry said, "and we have made it clear we are prepared to help them with that if they put themselves in line with the global community's requirements."
Kerry also made it clear that Kim Jong Un's continuing hostile language is "unacceptable," emphasizing that the United States and South Korea were beefing up their defense systems.
On Monday, CBS News correspondent David Martinthe U.S. Navy stationed a missile defense ship southwest of the Korean Peninsula, and a sea-going radar could soon be moved into position to keep close watch on North Korea's launch site for long-range missiles. The move came after the U.S. on Sunday sent F-22 fighter jets to South Korea in a military exercise. Last week, two radar-evading B-2 Spirit bombers were sent to practice runs over South Korea.
"The United States will defend our allies and that we will not be subject to irrational or reckless provocation," he said, adding: "No one takes it lightly, least of all the President of the United States, what has been happening."
The North's plutonium reactor was shut down and partially destroyed in 2007 as part of international nuclear disarmament talks that have since stalled. The declaration of a resumption of plutonium production - the most common fuel in nuclear weapons - and other facilities at the man Yongbyon (also spelled Nyongbyon) nuclear complex would boost fears in Washington and among its allies about North Korea's timetable for building a nuclear-tipped missile that can reach the United States, technology it is not currently believed to have.
The White House and the State Deparment said they were taking seriously the almost daily string of threats, but officials cast doubt on whether North Korea would follow through, portraying the latest threat as part of a pattern of antagonistic taunts that, so far, have not been backed up by action.
"There's a long way to go between a stated intention and actually being able to pull it off," said State Department spokeswoman Victoria Nuland.
Nuclear non-proliferation expert Mark Fitzpatrick, of the IISS, told CBSNews.com that Tuesday's warning from Pyongyang was merely "an announcement of an intention."
"It would take them at least 6 months to get it going again," said Fitzpatrick. "The coming weeks will probably provide some evidence as to whether they're following through on it."
That evidence, said Fitzpatrick, would likely come in the form of fresh satellite imagery showing work to restore a cooling system at the reactor. The cooling tower for the reactor was destroyed as part of a deal reached in the disarmament talks in 2007. According to Fitzpatrick, if the North Koreans have actively maintained the decommissioned facility since it was last visited by a Western scientist in 2010, then they could potentially get a new cooling system up and running within six months, and that activity would be spotted by satellites.
Fitzpatrick notes, however, that even the claim by North Korea that the facility will be restarted to enrich uranium and to produce plutonium for weapons represents, "a serious escalation to the threat that they pose."
Tuesday's announcement, said Fitzpatrick, is the first time the North Koreans have openly linked their uranium enrichment program to their weapons program.
Satellite imagery released in the spring and summer of 2012 showed new construction work taking place at the Yongbyon plant, including images which showed a new dome roof installed atop a separate, light-water reactor building. Analyst Allison Puccioni at IHS Jane's Defence Weekly said in a statement in August 2012 that an image taken by satellite earlier that month showed a new dome roof had been hoisted atop the reactor.
The unidentified spokesman for the North's atomic energy agency said Tuesday the move to bring the facility back online was part of efforts to resolve the country's acute electricity shortage but also for "bolstering up the nuclear armed force both in quality and quantity," according to a statement carried by the official Korean Central News Agency. He did not give a timetable for the resumption of work at Yongbyon.
Pyongyang conducted its third nuclear test in February, prompting a new round of U.N. sanctions that have infuriated its leaders and led to a torrent of threatening rhetoric. The United States has sent nuclear-capable bombers and stealth jets to participate in annual South Korean-U.S. military drills that the allies call routine but that Pyongyang claims are invasion preparations.
North Korea has declared that the armistice ending the Korean War in 1953 is void, threatened to launch nuclear and rocket strikes on the United States and, most recently, declared at a high-level government assembly that making nuclear arms and a stronger economy are the nation's top priorities.
The White House said Monday, however, that despite the bellicose rhetoric from North Korea, the Obama administration has not seen any significant changes in the regime's military posture.
White House press secretary Jay Carney said no military mobilization or repositioning of forces from Pyongyang to back up the threats had been detected.
The threats are seen, rather, primarily as efforts to force policy changes in Seoul and Washington and increase domestic loyalty to young North Korean leader Kim Jong Un by portraying him as a powerful military force.
"North Korea is keeping tension and crisis alive to raise stakes ahead of possible future talks with the United States," said Hwang Jihwan, a North Korea expert at the University of Seoul. "North Korea is asking the world, 'What are you going to do about this?"'
Nonetheless, the war rhetoric has reached new levels in the past weeks -- sufficient, reported CBS News foreign affairs analyst Pamela Falk, to prompt United Nations Secretary-Ban Ki-moon to warn in a statement and a tweet from the tiny European nation of Andorra that the crisis "has already gone too far."
"Nuclear threats are not a game," Ban said. "Aggressive rhetoric and military posturing only result in counter-actions, and fuel fear and instability. Things must begin to calm down, as this situation, made worse by the lack of communication, could lead down a path that nobody should want to follow."