A file handout satelite image provided by DigitalGlobe and dated September 29, 2004, shows the Yongbyon complex nuclear facility, some 100 km north of Pyongyang, North Korea. (DigitalGlobe /Via EPAkorea)

North Korea said Tuesday that it will resume operations of a shuttered nuclear reactor at its Yongbyon facility, allowing the secretive nation to potentially expand its stock of fissile material for nuclear weapons as it threatens war against the United States and its allies.

The brief announcement, carried by the state-run news agency, marked a notable escalation from the rhetoric of recent weeks, which has been fiery but until now backed up by little action.

The graphite-moderated reactor — along with several related facilities — had been shut down in 2007 as part of a diplomatic deal that sent heavy fuel oil to the North. Pyongyang said Tuesday it wanted to to restart the reactor and other nuclear facilities to ease the nation’s “acute shortage of electricity” and to bolster the “nuclear armed force both in quality and quantity.” Experts who have visited the Yongbyon facility say the small reactor is ill-suited for power production and geared instead to produce weapons-grade plutonium.

Plutonium can be culled from the spent fuel that is a byproduct of the reactor’s generating process. When the reactor is fully running, it can produce enough plutonium for about one bomb per year, according to atomic experts.

Restarting the reactor will take about six months, “unless they have been doing much of the preparatory work quietly,” Siegfried Hecker, a former director of the Los Alamos National Laboratory who has visited the North’s nuclear facilities numerous times, said in an e-mail.

U.N. Secretary General Ban Ki-moon said Tuesday he was “deeply troubled” by rising tensions on the Korean Peninsula and called for negotiations to resolve them.

“Nuclear threats are not a game,” he said at a news conference during a visit to Andorra. “Aggressive rhetoric and military posturing only result in counteractions and fuel fear and instability. Things must begin to calm down, as this situation, made worse by the lack of communication, could lead down a path that nobody should want to follow.”

Ban, a former South Korean foreign minister, said there was “no need” for North Korea “to be on a collision course with the international community.” He said he was sure that no nation intends to attack the North “because of disagreements about its political system or foreign policy. But he expressed fear “that others will respond firmly to any direct military provocation.” Ban added, “Dialogue and negotiations are the only way to resolve the current crisis.”

The North’s announcement comes in the wake of a Workers’ Party pledge that described nuclear weapons as the “nation’s life,” tradable not even for “billions of dollars.” The announcement also suggests that new leader Kim Jong Un is willing to expand his nation’s weapons program even if it upsets neighbors, including the North’s key benefactor, China, whose Foreign Ministry expressed “regret” about the decision to resume operations at Yongbyon.

A former U.S. official who has visited Yongbyon, speaking on condition of anonymity to speak frankly, said the North’s apparent decision is “very dangerous,” because the facilities are not monitored by outsiders.

“So what can we do to stop them?” the former official said. “They can get away with murder there, and we’re left on the outside jumping up and down.”

The five-megawatt reactor — the North’s only such facility — is the most visible symbol of North Korea’s three-decade mission to produce fissile material, necessary for any atomic weapon. But much of the North’s work has been done out of sight, likely in clandestine underground facilities that leave U.S. and South Korean officials with little insight about the North’s stockpile of weapons.

One key question is whether the North, since shutting down its reactor six years ago, has been making weapons with plutonium or uranium. If engineers are using plutonium, they are working from a dwindling stockpile — the mass produced from the reactor during its initial years of operation. But if engineers are using uranium, it means they are instead relying on a newer program — unveiled to Hecker during a 2010 visit — that involves thousands of centrifuges. Hecker was told those centrifuges were operational, but he wasn’t permitted to confirm it.

As part of its announcement, the North also pledged to “adjust and alter” the use of existing facilities, including its uranium-enrichment plant, which the North previously said was being used for peaceful energy purposes.

Experts say the North relied on plutonium for its first two nuclear tests in 2006 and 2009, but they remain uncertain about the material for its latest underground test in February. U.S. officials say the North took significant steps to prevent the release of radioactive traces from the blast site, which would have been detected by monitoring stations.

“My hunch is that the great efforts that they made to prevent us from finding out which type of fissile material they used indicate that they were in fact using the plutonium,” said Jacques Hymans, an associate professor at the University of Southern California who writes frequently about nuclear issues. “And if that is the case, then maybe they really need to restart the 5 [megawatt] reactor, diplomatic crisis or no crisis.”

Atomic experts think North Korea has enough leftover plutonium to make between four and eight bombs the size of those dropped on Hiroshima or Nagasaki during World War II.

When the North agreed to shutter its reactor as part of a multi-nation aid-for-disarmament deal, it demolished a 60-foot cooling tower, leaving a cloud of gray smoke. But the cooling tower, or a similar facility, is relatively easy to rebuild. The more pressing question is whether the North’s other plutonium facilities remain operational after being mothballed.

The reactor, first started up in 1986, was also frozen from 1994 until 2003 amid an earlier diplomatic agreement. Then-leader Kim Jong Il reactivated the reactor just as South Korea was breaking in a new president — as it is now, with Park Geun-hye having taken office in February.

During Hecker’s 2010 visit, the chief Yongbyon engineer told Hecker that the reactor was in “standby mode.”

“I told [the engineer] that people in the West claim it is beyond hope to restart” because it has been out of operation for so long, Hecker said. “He chuckled and said, ‘Yes, I know, that’s what they said in 2003 also, and they were wrong then as well.’ ”

Yoonjung Seo contributed to this report.