United Nations Secretary-General Ban Ki-moon is urging calm amid the latest rhetoric from North Korea, suggesting an impending war with the United States and South Korea.

"Nuclear threats are not a game," Ban said at a news conference in Andorra on Tuesday. "Aggressive rhetoric and military posturing only result in counter-actions, and fuel fear and instability.

"Things must calm down, as this situation, made worse by the lack of communication, could lead down a path that nobody should want to follow."

North Korea vowed on Tuesday to restart a nuclear reactor that can make one bomb's worth of plutonium a year, escalating tensions already raised by near daily warlike threats.

The North's plutonium reactor was shut down in 2007 as part of international nuclear disarmament talks that have since stalled. The declaration of a resumption of plutonium production — the most common fuel in nuclear weapons — and other facilities at the main Nyongbyon nuclear complex will boost fears in Washington and among its allies about North Korea's timetable for building a nuclear-tipped missile that can reach the United States, technology it is not currently believed to have.

300-korea-tanks-rtxy5gg

South Korean soldiers conduct field firing drills near the demilitarized zone separating the two Koreas in Paju, north of Seoul on Tuesday after North Korea's official news agency announced it will restart its shuttered Yongbyon nuclear reactor. Pyongyang's move that could enable it to extract more plutonium to bolster its atomic stockpile for use in nuclear weapons. (Lee Jae-Won/Reuters)

A spokesman for the North's General Department of Atomic Energy said that scientists will begin work at a uranium enrichment plant and a graphite-moderated five-megawatt reactor, which generates spent fuel rods laced with plutonium and is the core of the Nyongbyon nuclear complex.

The unidentified spokesman said the measure is part of efforts to resolve the country's acute electricity shortage but also for "bolstering up the nuclear armed force both in quality and quantity," according to a statement carried by the official Korean Central News Agency.

Pyongyang conducted its third nuclear test in February, prompting a new round of UN sanctions that have infuriated its leaders and led to a torrent of threatening rhetoric. The United States has sent nuclear-capable bombers and stealth jets to participate in annual South Korean-U.S. military drills that the allies call routine but that Pyongyang claims are invasion preparations.

North Korea has declared that the armistice ending the Korean War in 1953 is void, threatened to launch nuclear and rocket strikes on the United States and, most recently, declared at a high-level government assembly that making nuclear arms and a stronger economy are the nation's top priorities.

The threats are seen as efforts to force policy changes in Seoul and Washington and increase domestic loyalty to young North Korean leader Kim Jong-un by portraying him as a powerful military force.

"North Korea is keeping tension and crisis alive to raise stakes ahead of possible future talks with the United States," said Hwang Jihwan, a North Korea expert at the University of Seoul. "North Korea is asking the world, 'What are you going to do about this?" '

North Korea added the five-megawatt, graphite-moderated reactor to its nuclear complex at Nyongbyon in 1986 after seven years of construction. The country began building a 50-megawatt and a 200-megawatt reactor in 1984, but construction was suspended under a 1994 nuclear deal with Washington.

North Korea says facility is for electricity

North Korea says the facility is aimed at generating electricity. It takes about 8,000 fuel rods to run the reactor. Reprocessing the spent fuel rods after a year of reactor operation could yield about seven kilograms of plutonium — enough to make at least one nuclear bomb, experts say.

Nuclear bombs can be produced with highly enriched uranium or with plutonium. North Korea is believed to have exploded plutonium devices in its first two nuclear tests, in 2006 and 2009.

In 2010, the North unveiled a long-suspected uranium enrichment program, which would give it another potential route to make bomb fuel. Uranium worries outsiders because the technology needed to make highly enriched uranium bombs is much easier to hide than huge plutonium facilities.

But experts say plutonium is considered better for building small warheads, which North Korea needs if it is going to put them on missiles. Analysts say they don't believe North Korea currently has mastered such miniaturization technology.

Enough for 4 nuclear bombs

Scientist and nuclear expert Siegfried Hecker has estimated that Pyongyang has 24 to 42 kilograms of plutonium — enough for perhaps four to eight rudimentary bombs similar to the plutonium weapon used on Nagasaki in the Second World War.

It's not known whether the North's latest atomic test, in February, used highly enriched uranium or plutonium stockpiles. South Korea and other countries have so far failed to detect radioactive elements that may have leaked from the test and which could determine what kind of device was used.

"North Korea is dispelling any remaining uncertainties about its intention for developing nuclear arms. It is making it clear that its nuclear arms program is the essence of its national security and that it's not negotiable," said Sohn Yong-woo, a professor at the Graduate School of National Defence Strategy of Hannam University in South Korea.

"North Korea is more confident about itself than ever after the third nuclear test," Sohn said. "That confidence is driving the leadership toward more aggressive nuclear development."