Seoul's defence ministry said the North is believed to have some 50kg of weapons-grade plutonium as of the end of last year - enough to make about 10 weapons - up from 40kg eight years earlier.
In a biannual report, the ministry also estimated that Pyongyang has a significant capacity of producing weapons using enriched uranium.
A 2016 assessment released by the Pentagon's weapons testing office on Tuesday said that US ground-based interceptors meant to knock out any incoming ICBM still had low reliability, giving the system itself a limited capability of shielding the United States.
The United Nations Security Council adopted two resolutions against the North previous year, following an unprecedented pair of nuclear tests and multiple rocket and missile launches.
But it did not estimate weapons-grade uranium stocks, citing impenetrable secrecy in the North's uranium programme.
North Korea deactivated the Yongbyon reactor in 2007 under an aid-for-disarmament accord, but began renovating it after Pyongyang's third nuclear test in 2013.
Via its biennial defense white paper, Seoul for the first time directly highlighted the potential danger posed by North Korean intercontinental ballistic missiles (ICBM) and submarine-launched ballistic missiles (SLBM).More news: Officer shot, killed in Orlando, suspect still at large
Analysts are split on how close North Korea is to achieving its nuclear goals, but all agree that it has made large strides since Kim took over as leader after his father Kim Jong Il died in December 2011.
A ministry official explained that the latest evaluation is based on the timing of the reactivation and stoppage of reactor operations, the scale of spent fuel reprocessing and how much plutonium is consumed at nuclear experiments in the North.
Kim Jong-Un said during his New Year speech that his country is on the verge of testing an intercontinental ballistic missile.
The previous week, Trump made a statement via Twitter that the development was "not going to happen", and vowed to halt Pyongyang in its tracks.
The recognition of North Korea's nuclear strength comes at a bad time domestically, with South Korean President Park Geun-gye now going through an impeachment trial over a power abuse scandal.
The ministry also said the country developed a hydrogen bomb and "had access to standardized nuclear warheads by bolstering up its nuclear weapons on a high level at an unimaginably high speed on the strength of self-development" despite the global sanctions.