"I think that there has to be some very strong steps and safeguards," he said, noting that the North's ability to build a nuclear bomb can never be taken away once it has the knowledge. "I think President Trump has met with Kim Jong-un twice now. It hasn't really been brought to fruition. And I don't particularly think it's helpful to meet with him again unless we see that there are very positive ways where they have discarded their nuclear weapons and discarded the ability to make them again."