North Korea has expanded its nuclear arsenal to 30 warheads and is expected to increase further as Pyongyang produces more weapons-grade plutonium and uranium, according to recent estimates.

Moreover, North Korea will control enough fissile material to increase the arsenal number twofold to as many as 60 weapons by 2020, the Institute of Science and International Security in Washington said.

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The rapid increase is alarming as US previously estimated in 1999 that Pyongyang only owned just one or two nuclear weapons and would only increase up to 10 or more in 2020, Washington Times reported citing a secret Defense Intelligence Agency report.

 "The bottom line is that North Korea has an improving nuclear weapons arsenal... The last several years have witnessed a dramatic and overt buildup in North Korea's nuclear weapons capabilities," David Albright, the institute's founder and director, told the Times.

That being said, North Korea is beginning to emerge as a world nuclear power and has the capability to initiate a strike to neighboring regions. However, Albright noted that although Pyongyang could "theoretically use a satellite launcher" to attack the US, it is "not with any reliability."

He added that Pyongyang's ability to create a reliable and survivable warhead for intercontinental ballistic missiles (ICBMs) remains uncertain. But the country has likely mastered the engineering to place a smaller nuclear warhead on its short-range Nodong missile that could strike South Korea or Japan. It is not clear if North Korea has attained the same success for its longer-range ballistic missiles.

Meanwhile, tensions between the US and North Korea have reached boiling point as the latter is nearing to conduct its sixth nuclear test, a move that would likely lead to open conflict. US officials said in an interview with the NBC last week that Washington is considering of launching a preemptive conventional strike as Pyongyang's nuclear test looms. North Korea, on the other hand, has previously threatened to reduce the US "to ashes" if a preemptive strike is conducted.