WASHINGTON » The Obama administration on today warned North Korea to halt a recent spate of "unacceptable" rhetoric and actions that Secretary of State John Kerry called "provocative, dangerous and reckless." Kerry also vowed that the United States would defend itself and its allies South Korea and Japan from North Korean threats.

"We have heard an extraordinary amount of unacceptable rhetoric from the North Korean government in the last few days," Kerry told reporters at a joint news conference with visiting South Korean Foreign Minister Yun Byung-se.

"The bottom line very simply is that what Kim Jong-Un has been choosing to do is provocative, it is dangerous, reckless, and the United States will not accept the DPRK as a nuclear state," Kerry said, referring to North Korea’s young new leader and the country’s official name, the Democratic People’s Republic of Korea.

"The United States will do what is necessary to defend us and ourselves and our allies," he said. "We are fully prepared and capable of doing so and the DPRK understands that."

His comments came as North Korea ratcheted up an almost daily string of threats toward the U.S., South Korea and Japan with an announcement earlier today that it would revive a long-dormant nuclear reactor and ramp up production of atomic weapons material.

A North Korean official said the country would quickly begin "readjusting and restarting" the facilities at its main Nyongbyon nuclear complex, including the plutonium reactor and a uranium enrichment plant, which was shuttered as part of international nuclear disarmament talks in 2007 that have since stalled.

Kerry said such a step would be "a direct violation" of North Korea’s international commitments.

State Department spokeswoman Victoria Nuland said that development would be "extremely alarming" while the White House said President Barack Obama’s entire national security team was focused on North Korea.

However, U.S. officials did 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," Nuland said.

Still, the Pentagon suggested the administration is concerned about the prospect for further escalation of tensions and it has made a conspicuous display of firepower in recent weeks, sending B-52 and B-2 bombers on practice runs over South Korea, as well as deploying F-22 stealth fighters and repositioning a missile-defense ship off the Korean coast.

These moves and others are meant to deter North Korea from launching even a limited military strike against the South, while also offering reassurance to Seoul that the U.S. will stick to its treaty obligation to defend the South against attack.

"We are looking for the temperature to be taken down," Pentagon spokesman George Little told reporters today. "We are in the business of assuring our South Korean allies that we will help defend them in the face of threats."

At the White House, press secretary Jay Carney called on Russia and China, two countries he said have influence on North Korea, to use that influence to persuade the North to change course.

North Korea’s recent tide of nuclear vows and aggressive threats are seen as efforts to force Washington into disarmament-for-aid talks and to boost young leader Kim Jong Un’s stature as a strong military leader. Pyongyang has reacted angrily to U.S.-South Korean military drills and a new round of U.N. and U.S. sanctions that followed North Korea’s Feb. 12 underground nuclear test.

Although world leaders have largely shrugged off the threats as more of the same from North Korea, U.N. Secretary-General Ban Ki-moon said today that the North appears to be "on a collision course with the international community," adding that the current crisis has gone too far

AP National Security Writer Robert Burns and AP writer Matthew Pennington contributed to this report.