Seoul, meanwhile, on Monday warned that any provocative moves from North Korea would trigger a strong response "without any political considerations."
The motivation behind the North's announcement Tuesday on the nuclear facilities was unclear, said Ramesh Thakur, director of the Center for Nuclear Nonproliferation and Disarmament at Australian National University in Canberra, suggesting that it was unlikely to make a big difference militarily for the country, which is already believed to have four to 10 nuclear weapons.
The North Koreans may be hoping to use the move as a bargaining chip in any future talks, he said, or it could be an attempt by the country's young leader to shore up support domestically.
"It's just a very murky situation," Thakur said. "The danger is that we can misread one another and end up with a conflict that no one wants."
China, a key North Korean ally, expressed regret over Pyongyang's announcement about the reactor.
"China has consistently advocated denuclearization on the peninsula and maintaining peace and stability in the region," Chinese Foreign Ministry Spokesman Hong Lei said Tuesday at a regular news briefing.
Japanese Chief Cabinet Secretary Yoshihide Suga said the move would need to be dealt with in a serious manner, noting that it breached the North's previous commitments.
On Tuesday, Kerry refused to speculate about North Korea's intentions or what its strategy may be with regard to its plans to reopen its reactor.
"We've heard an extraordinary amount of unacceptable rhetoric from the North Korean government in the last days. So let me be perfectly clear here today: The United States will defend and protect ourselves, and our treaty ally, the Republic of Korea," he said.
Kerry reiterated the U.S. policy with regard to North Korea, saying the United States believes there is "a very simple way" for Pyongyang to end the sanctions by ending its nuclear ambitions.
Kerry was scheduled to visit Seoul next week, while South Korea's president was due in Washington for talks with President Barack Obama.
A torrent of threats
The North's latest declaration comes after a stream of verbal attacks against South Korea and the United States in recent weeks, including the threat of a nuclear strike.
Pyongyang's angry words appear to have been fueled by the recent joint military exercises by the United States and South Korea in the region, as well as tougher U.N. sanctions in response to North Korea's latest nuclear test in February.
Much of the bellicose rhetoric, analysts say, isn't matched by the country's military capabilities.
The North's announcement Tuesday follows a new strategic line "on simultaneously pushing forward economic construction and the building of the nuclear armed force." It was announced Sunday during a meeting of a key committee of the ruling Workers' Party of Korea headed by Kim Jong Un.
The work of adapting and restarting the nuclear facilities "will be put into practice without delay," KCNA said.
The measures would help solve "the acute shortage of electricity," as well as improving the "quality and quantity" of the country's nuclear arsenal, it said.
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