Experts say North Korea is probably years away from perfecting the technology to back up its bold threats of a pre-emptive nuclear strike on America, but some aren't sure about nearby targets like South Korea and Japan.

North Korean President Kim Jong Un Kim Jong Un inspects various military exercises (YouTube)

David Albright at the Institute for Science and International Security assesses that the North could put a nuclear warhead on an 800-mile-range missile. But scientist Siegfried Hecker who has had unusual access there, says it is unlikely and nobody outside of a small elite would know.

The bottom line is that no one can say with certainty how much technological progress North Korea has made. Even if it has the capability, it's unlikely Pyongyang would use it because the retaliation would be devastating.

More SKoreans leave NKorean factory park under ban

More South Koreans have begun to leave North Korea and the factory park where they work, four days after Pyongyang closed the border to people and goods.

Seoul's Unification Ministry says seven South Koreans left the Kaesong industrial park Saturday morning, and about 100 of the roughly 600 still there were expected to return home by day's end.

One manager interviewed as he left says North Korea has bolstered security along the border.

The industrial park is the last remnant of North-South cooperation. Pyongyang's blocking of traffic there is among many provocative moves it has made recently in anger over U.N. sanctions for its Feb. 12 nuclear test and current U.S.-South Korean military drills. North Korea suggested earlier this week that diplomats in Pyongyang leave for their own safety.