A nuclear expert who spoke with South Korea's Yonhap news agency on condition of anonymity said it appeared Pyongyang has spent "at least $6.58 billion" on its nuclear program.
North Korea, he said, might be capable of producing around 90 pounds "of highly enriched uranium per year, enough for one or two atomic weapons."
North Korea last month declared a moratorium on nuclear activity and long-range missile tests in exchange for substantial food assistance from the United States. The deal was scrapped, however, after North Korea tried to send an Earth observation satellite into orbit using a long-range rocket.
Japanese Prime Minister Yoshihiko Noda, in Washington this week, said Tokyo was concerned North Korea might be on the verge of testing a nuclear device. A series of missile launches by North Korea in 2009 coincided with the underground testing of a nuclear device.
Tensions on the Korean Peninsula have escalated since the December death of North Korean leader Kim Jong Il.