SEOUL, South Korea — North Korea will keep improving its nuclear and long-range ballistic missile capabilities next year to gain leverage to force Washington to make concessions, like the easing of sanctions, government and private analysts in South Korea said on Tuesday.

The isolated North has made major strides this year in its nuclear weapons program but has also faced increasingly tough sanctions from the United Nations. On Sept. 3, it detonated what it called a hydrogen bomb in its sixth and most powerful nuclear test. It has also launched three intercontinental ballistic missiles this year, demonstrating rockets powerful enough to deliver warheads to all of the continental United States.

After its last ICBM test, conducted Nov. 29, North Korea claimed to have completed building its nuclear force. But the country has yet to clear a key technological hurdle: proving that its warheads can survive re-entry into the atmosphere after flying through space, according to Western officials and analysts.

Forecasting North Korea’s nuclear weapons program for the new year, the Unification Ministry in South Korea said on Tuesday that the North “will continue to advance its nuclear and missile capabilities” in 2018 despite its claims about completing its nuclear force.