North Korea's state-run Korean Central News Agency reported Tuesday the country's atomic energy department would "readjust and restart all the nuclear facilities" at its main nuclear complex in Yongbyon.
North Korea's announcement was followed by a plea for calm from Ban, a South Korean, who said he was "deeply troubled," CNN reported.
"The current crisis has already gone too far," he said in a statement. "Nuclear threats are not a game. Aggressive rhetoric and military posturing only result in counter-actions, and fuel fear and instability."
In calling for calm, Ban said he didn't want the situation to worsen by a lack of communication that "could lead down a path that nobody should want to follow."
Dialogue and negotiations are "the only way to resolve the current crisis," Ban said.
The Yongbyon plant includes a uranium enrichment facility and a reactor that were shuttered and disabled under a 2007 agreement during talks among North Korea, the United States and four other nations.
The state-run news agency said work on the facility would begin "without delay" but offered no time table.
Tuesday's announcement is part of a new strategy of pushing economic construction and building a nuclear armed force announced Sunday by leader Kim Jong Un, CNN said.
Recent tensions -- fueled by international criticism and a new round of U.N. sanctions over a missile launch and the third nuclear test conducted by North Korea -- have led the leaders Pyongyang to sever a key military hotline with Seoul and declare void the 1953 armistice that stopped the Korean War, among other things.
The United States has made a show of its military strength during its joint annual training exercises with South Korea, flying B-2 stealth bombers that can carrying conventional or nuclear weapons, as well as B-52s and F-22 Raptor stealth fighters over South Korea.
Also, the United States moved a sea-based radar platform nearer to the North Korean coast to monitor military moves, military sources in Seoul told Yonhap Tuesday.
The U.S. military sent the Sea-Based X-Band Radar, based in Hawaii, to the western Pacific Ocean to better monitor any potential attempts from North Korea to launch a long-range missile, the officials said.
The radar system also can search and track targets, as well as communicate with interceptor missiles at its overseas base that can shoot down a target missile.
On Monday, South Korean officials warned that any provocative moves from its northern neighbor would set off a strong response "without any political considerations."
China, a key North Korean ally, expressed regret over the announcement about the reactor at Yongbyon, CNN said.
"China has consistently advocated denuclearization on the peninsula and maintaining peace and stability in the region," Chinese Foreign Ministry Spokesman Hong Lei said Tuesday during a news briefing.
Japanese Chief Cabinet Secretary Yoshihide Suga said the decision would need to be addressed seriously.
A senior South Korean government official said North Korea's new party line eliminated any lingering "ambiguity" over what it might try to do with its nuclear weapons, The New York Times reported.
"We now know their real intention. The picture is clear. What we will do is the combined will of the international community," the official said.
The official, who spoke on condition of anonymity, said South Korea, the United States and their allies must use "all means" to pressure North Korea, including investigations into the country's human rights abuses as well as economic sanctions.
"They are depending on nuclear weapons for their survival, but we must persuade them that there is an alternative and brinkmanship doesn't work."
South Korea's news agency Yonhap reported North Korea mothballed the reactor but started reprocessing 8,000 fuel rods stored at the site in 2009 after it didn't get the energy assistance it had wanted in exchange for shuttering the facility.
Yonhap, quoting experts, said North Korea probably used plutonium as fissile material for nuclear tests in 2006 and 2009. However, it isn't clear if plutonium or uranium was used in the Feb. 12 test.