"The long term security for South Korea will be undermined severely if North Korea gets that deliverable nuclear weapons capability," he said.

Last week, Adm. Harry Harris, chief of the US Pacific Command, described the North's aims in similar terms.

Harris said he does not agree with the predominant view that North Korean leader Kim Jong-un is pursuing nuclear-tipped missiles to safeguard his regime only. The commander told a congressional hearing that Kim is "after reunification under a single, communist system" and that that is the "long view."

Central Intelligence Agency Director Mike Pompeo and Director of National Intelligence Dan Coats made similar remarks at a separate hearing.

Bolton served as under secretary of state for arms control and international security affairs from 2001-2005. At the time, he made a series of remarks highly critical of the regime in Pyongyang and then-leader Kim Jong-il. He also called for a regime change in Pyongyang.

The North unleashed vituperation in response, describing him as "human scum," "devoid of reason," "an ugly fellow who cannot be regarded as a human being" and a "bloodthirsty fiendish bloodsucker," among other things. (Yonhap)