North Korea could build an arsenal of up to 48 nuclear weapons, several times more than it has now, if the communist state were able to step up its program unchecked, a U.S. think tank said Thursday.

North Korea 's nuclear program is shrouded in secrecy, but the Institute for Science and International Security used limited available information to chart out different scenarios for Pyongyang 's progress in the coming years.

North Korea, which first tested an atomic bomb in 2006, is believed to be able to produce between six and 18 nuclear weapons through a plutonium program. It is less clear whether it has the means to deliver the weapons.

With international negotiations at a standstill, satellite imagery has shown that North Korea has been pursuing work on a light-water reactor that is officially for civilian purposes but could produce weapons-grade plutonium.

Kim Jong Un's regime is also constructing a uranium enrichment plant, ostensibly to produce fuel for the new reactor. But scientists believe North Korea could also use the plant to develop highly enriched uranium, giving the country a second way to produce nuclear weapons.

The study found that, if the light-water reactor doesn't produce plutonium for nuclear weapons, the growth in North Korea 's arsenal will be modest, with 14 to 25 nuclear weapons by the end of 2016.

But if North Korea dedicates the light-water reactor to weapons-grade plutonium and also produces weapons-grade uranium, it could have 28 to 39 nuclear weapons by the end of 2016, the study said.

The figure would go up to 37 to 48 nuclear weapons if North Korea also secretly has a second centrifuge plant for uranium, as some experts believe.

"As in many other cases, negotiations are the best way to alleviate the security challenges posed by North Korea 's nuclear program. They should be pursued vigorously," said the report, written by experts David Albright and Christina Walrond.

But the study said that the U.S. otherwise had few options other than to try to hamper progress.

The authors said that one way to slow North Korea 's program was to persuade China -- Pyongyang 's primary partner--to clamp down on smuggling and enforce sanctions.

North Korea agreed in 2005 and 2007 to end its nuclear program in return for aid and security guarantees, but agreements with Pyongyang are notoriously short-lived.

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