Story highlights South Korea's unification minister held an unusually candid news conference Tuesday

"They have been developing their nuclear capabilities faster than expected," he said

Seoul, South Korea (CNN) North Korea may be able to pair a nuclear warhead with a long-range ballistic missile sometime next year, a top South Korean minister said during an unusually candid assessment of Pyongyang's weapons programs.

Authorities in Seoul believe North Korean leader Kim Jong Un will push to achieve that goal in time for next September's 70th anniversary of the founding of the Democratic People's Republic of Korea, as North Korea is officially known, Unification Minister Cho Myoung-gyon said Tuesday at a news conference.

"They have been developing their nuclear capabilities faster than expected," Cho said. "We cannot rule out the possibility of North Korea declaring the completion of their nuclear program next year."

A US official speaking on the condition of anonymity told CNN over the summer that Washington believes Pyongyang will be able to launch a reliable nuclear-capable intercontinental ballistic missile (ICBM) by early 2018.

Lt. Gen. Vincent Stewart, head of the US Defense Intelligence Agency, said in a hearing before the Senate Armed Services Committee in May that North Korea's final hurdle is perfecting technology to ensure a nuclear warhead survives the heat-intensive process of reentering the earth's atmosphere.

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