Nuclear scientist Siegfried Hecker who visited clandestine nuclear facilities in North Korea last year says the North may launch a third nuclear test to allow it to develop a warhead with the potential to be carried by a missile.

The Stanford University professor said at a lecture Friday that North Korea's second test in 2009 was necessary because their first attempt in 2006 "didn't work well."

He said that if Pyongyang wants to prove that it can develop a small, technically advanced missile warhead "they would need to have one more nuclear test."

Hecker was invited to North Korea last year where he was shown a modern uranium enrichment plant in Yongbyon that the North had previously denied possessing.

Hecker is a professor in the Department of Management Science and Engineering at Stanford.