"North Korea has ample technical reason to test again as it moves toward developing a credible nuclear-tipped ICBM. The test site appears to be ready," Hecker said. "I believe the only thing that inhibits them from testing is the political fallout they would face, particularly from China and the new South Korean administration."

Hecker said that the North, through five nuclear tests since 2006, is believed to have learned to make nuclear warheads small and light enough to mount on smaller missiles that can reach targets anywhere in South Korea and Japan.

He also said the North is estimated to have enough plutonium and highly enriched uranium for 20 to 25 nuclear weapons.

"All estimates are to a large extent based on the observations my Stanford colleagues and I made during my last visit to Yongbyon in November 2010. To my knowledge, no outsiders have been in their nuclear complex since," he said. "The rest of the estimate is based on indirect evidence ? that is, satellite imagery, and what North Korea chooses to publicize, combined with modeling of their capabilities and acquisitions."

Noting that President Moon favors diplomatic engagement with the North, Hecker said it is imperative for Seoul and Washington to "craft a unified strategy on North Korea and speak with one voice."

"That voice should reflect the new South Korean president's views on finding a diplomatic resolution," he said. "I hope that the Trump administration will support President Moon, and that Pyongyang won't pre-empt diplomacy with a nuclear or long-range missile test."

Hecker also said he would suggest Trump send an envoy to Pyongyang.

"There is a danger that overconfidence or miscalculation by Kim Jong-un, or an unpredictable reaction to a crisis, could result in a nuclear detonation. I also have serio concerns about a nuclear-weapon accident in North Korea, particularly if Pyongyang feels threatened and begins to deploy its nuclear arsenal," he said. (Yonhap)