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.
Yongbyon's back story
In June 2008, the usually secretive North Korean government made a public show of destroying the cooling tower of the Yongbyon reactor to demonstrate its compliance with a deal to disable its nuclear facilities.
But two months later, as its then-leader, Kim Jong Il, balked at U.S. demands for close inspections of its nuclear facilities, the North started to express second thoughts.
It said it was suspending the disabling of its nuclear facilities and considering steps to restore the facilities at Yongbyon "to their original state."
In November 2009, it announced it was reprocessing nuclear fuel rods as part of measures to resume activities at Yongbyon. It noted success in turning the plutonium it had extracted into weapons-grade material.