WASHINGTON (AP) — Secretary of State John Kerry on Tuesday warned North Korea to halt a recent spate of rhetoric and actions, calling them provocative, dangerous and reckless. He also vowed that the United States would defend itself and its allies South Korea and Japan from North Korean threats.

Kerry's comments came after North Korea ratcheted up an almost daily string of threats toward the three nations with an announcement that it would revive a long-dormant nuclear reactor and ramp up production of atomic weapons material.

Speaking to reporters at a joint news conference with South Korean Foreign Minister Yun Byung-se, Kerry said the Democratic People's Republic of Korea, or DPRK, knows that the U.S. is fully prepared and capable of defending itself and its allies.

"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.

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. It had been 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 and a "very serious step."

"It would be a provocative act and completely contrary to the road we have traveled for all these years," he said.

Still, both Kerry and his South Korean counterpart said the door remained open for North Korea to return to multi-national nuclear disarmament talks.

Yun said those talks remain a "useful tool" for getting North Korea to give up its nuclear weapons, although he conceded it would be a very difficult task. "We should continue these efforts," he said.

"If North Korea decides to give up its nuclear ambitions and to become a member of the international community, we are prepared to resume talks" for peace on the Korean Peninsula, he said.

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Yun said South Korean President Park Geun-hye is open to building a trusting relationship with North Korea but that Seoul would respond to provocations from Pyongyang. It was critical that the U.S. and South Korea continue to enhance their defense capabilities, he said.

The White House said President Barack Obama's entire national security team was focused on North Korea, although some U.S. officials did cast doubt on whether North Korea would follow through on its threat to restart the reactor, portraying the latest threat as part of a pattern of antagonistic taunts that, so far, have not been backed up by action.

State Department spokeswoman Victoria Nuland said that development would be "extremely alarming" but added: "There's a long way to go between a stated intention and actually being able to pull it off."

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. "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 with 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 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 Tuesday that the North appears to be "on a collision course with the international community," adding that the current crisis has gone too far.

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