North Korea has assembled an estimated 50 warheads and has fissile material to produce up to 40 more, according to the Stockholm International Peace Research Institute.

"An ICBM capable of reaching the United States is almost fully developed, and they are continuing to build the capacity to produce approximately 10 to 20 nuclear bombs per year," President Lee Jae Myung said at the Center for Strategic and International Studies.

Referring to the harder-line policies of his predecessor and lack of diplomacy with North Korea, Lee said: "We have made efforts to deter North Korea and apply sanctions, but the result has been North Korea continues developing its nuclear program."

"The hard fact is that the number of nuclear weapons that North Korea possesses has increased over the past three to four years," he said.

Lee said that South Korea was committed to conventional weapons deterrence against the North but also pointed to his efforts to ease steps seen as provocative, such as ending loudspeaker blasting of anti-North Korea messages across the military frontier.

He was speaking after talks with President Donald Trump, who said he hoped to meet with North Korean leader Kim Jong Un again.