North Korean leader Kim Jong Un inspects a military factory in this undated picture released by the Korean Central News Agency on Friday, May 17. North Korea launched several short-range guided missiles into the sea off the Korean Peninsula's east coast May 18, South Korea's semi-official news agency Yonhap cited the South Korean Defense Ministry as saying.
(CNN) -- North Korea's latest nuclear test, coming on the heels of December's successful satellite launch, suggests that Pyongyang is moving forward toward developing a nuclear warhead and a deliverable missile system, experts say. The question remains: How close is it?
The answer, like the cloistered "hermit kingdom," remains largely a mystery, as does much of its nuclear program.
"It's a question over the delivery system and the reliability of those systems," said Daniel Pinkston, senior analyst for the International Crisis Group covering Northeast Asia. "That is essentially unknown, or known by a few people inside North Korea."
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A 2009 report by International Crisis Group suggests that North Korea "probably has somewhere between six and twelve nuclear weapons, or at least explosive devices," but notes that experts are divided on whether any of these are now usable as warheads -- small enough to be mounted on missiles and durable enough to withstand the hazards of flight.
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"It's pretty clear that these are advanced technologies and the systems present a number of engineering challenges -- and to master these technologies requires a number of tests," Pinkston said.
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In December, on the first anniversary of the death of its former leader Kim Jong Il, North Korea successfully launched a three-stage rocket that put the satellite Shining Star-3 into orbit. The launch also signaled that the North's long-range missile program now puts the United States within reach.
Last month, the United Nations Security Council unanimously passed a resolution that strengthened sanctions against the North in response to the December rocket launch. Declaring sanctions to be tantamount to "a declaration of war," North Korea threatened further missile and nuclear tests, which it said are a new phase of confrontation with the United States.
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Tuesday's underground nuclear test is North Korea's third, following tests in 2009 and 2006. The test, "probably" conducted in the vicinity of P'unggye, yielded "several kilotons," according to assessments cited by the U.S. director of national intelligence.
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Estimates of the size, or yield, of the 2009 nuclear test range from 2.5 kilotons to 6 kilotons, Pinkston said. By comparison, the atomic bomb dropped on Hiroshima had a yield of 16 kilotons.
While the specter of a North Korea able to send nuclear-tipped missiles is worrisome, equally troublesome to the international community is Pyongyang's atomic technology fueling the black market for weapons.
"If its clandestine uranium-enrichment program has made strides, Pyongyang could demonstrate that it will gain access to a far larger pool of fissile material than simply its limited supply of weapons-grade plutonium," wrote Patrick M. Cronin, an Asia expert at the Center for New American Security, in a CNN op-ed. "A larger pool of fissile material is a dual threat: As a vital part of an expanded nuclear weapon program and as a commodity to be sold on the black market."
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CNN's Jethro Mullen contributed to this report.
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Kim Jong Un visits the Ministry of People's Security on Wednesday, May 1, as part of the country's May Day celebrations.
A North Korea soldier gestures to stop photographers from taking photos from a Chinese tour boat as other soldiers look on along the North Korean bank of the Yalu River near the town of Sinuiji across the Chinese city of Dandong in Liaoning province, China, on Saturday, April 6.
North Korean soldiers gather by the docks in Sinuiju near the Chinese border on Thursday, April 4.
North Korean soldiers patrol along the Yalu River in Sinuiju across the border from the Chinese city of Dandong on April 4.
Kim Jong Un is briefed by his generals in this undated photo. On the wall is a map titled "Plan for the strategic forces to target mainland U.S."
Kim Jong Un works during a briefing in this undated photo.
North Korean leader Kim Jong Un inspects drills by the Korean People's Army navy at an undisclosed location on North Korea's east coast on March 25 in a photo from the state-run Korean Central News Agency.
Kim makes his way to an observation post with North Korean soldiers on March 25.
Kim uses a pair of binoculars to look south from the Jangjae Islet Defense Detachment near South Korea's Taeyonphyong Island on March 7.
Kim is greeted by the family of a soldier as he inspects Jangjae Islet Defense Detachment near South Korea's Taeyonphyong Island in South Hwanghae province on Thursday, March 7, in a photo from the state-run Korean Central News Agency.
Kim is surrounded by soldiers during a visit to the Mu Islet Hero Defense Detachment near South Korea's Taeyonphyong Island on March 7. North Korea has escalated its bellicose rhetoric, threatening nuclear strikes, just before the U.N. Security Council passed tougher sanctions against the secretive nation on March 7.
Kim arrives at Jangjae Islet by boat to meet with soldiers of the Jangjae Islet Defense Detachment near Taeyonphyong Island in South Hwanghae province on March 7.
Soldiers in the North Korean army train at an undisclosed location on March 6.
North Korean leader Kim Jong Un, center, poses with chiefs of branch social security stations in this undated picture released by North Korea's official news agency on November 27.
Kim celebrates with staff from the satellite control center in Pyongyang, North Korea, during the launch of a rocket carrying a satellite, in a photo released by the official North Korean news agency on December 12.
A crowd watches as statues of the nation's founder, Kim Il Sung, and his son Kim Jong Il are unveiled during a ceremony in Pyongyang on April 13, 2012. Photos from North Korea are rare, but the country was on full display in April 2012 as it celebrated the 100th birthday of Kim Il Sung.
A North Korean soldier stands guard in front of an UNHA III rocket at the Tangachai-ri Space Center on April 8, 2012.
In April 2012, Pyongyang launched a long-range rocket, which broke apart and fell into the sea. The UNHA III rocket is pictured on its launch pad in Tang Chung Ri, North Korea.
A closer look at the UNHA III rocket on its launch pad in Tang Chung Ri, North Korea.
A military vehicle participates in a parade in Pyongyang on April 15, 2012.
North Koreans wave flags in front of portraits of Kim Il Sung, left, and his son Kim Jong Il during celebrations to mark the 100th birth anniversary of Kim Il Sung in Pyongyang on April 16, 2012.
North Korean soldiers relax at the end of an official ceremony attended by leader Kim Jong Un at a stadium in Pyongyang on April 14, 2012.
Kim Jong Un applauds as he watches a military parade in Pyongyang on April 15, 2012.
A North Korean soldier stands on a balcony in Pyongyang on April 16, 2012.
North Korean soldiers march during a military parade in Pyongyang on April 15, 2012.
Soldiers board a bus outside a theater in Pyongyang on April 16, 2012.
North Korean performers sit below a screen showing images of leader Kim Jong Un in Pyongyang on April 16, 2012.
North Korean soldiers salute during a military parade in Pyongyang on April 15, 2012.
Kim Jong Un visits the Rungna People's Pleasure Ground, which is under construction in Pyongyang, in a photo released on July 3, 2012, by North Korea's official Korean Central News Agency.
North Korean soldiers listen to a speech during an official ceremony attended by leader Kim Jong Un at a stadium in Pyongyang on April 14, 2012.
Members of a North Korean military band gather following an official ceremony at the Kim Il Sung stadium in Pyongyang on April 14, 2012.
North Korean military personnel watch a performance in Pyongyang on April 16, 2012.
A North Korean controller is seen along the railway line between Pyongyang and North Pyongan province on April 8, 2012.
A North Korean military honor guard stands at attention at Pyongyang's airport during a diplomatic visit on May 2, 2001.
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