North Korea could conduct its fourth nuclear test in weeks or months, said Dr. Siegfried Hecker from Stanford University, the last U.S. expert to visit the Yongbyon uranium enrichment facility, according to Radio Free Asia on Friday.

He made the comments during a seminar at the Vienna Center for Disarmament and Non-Proliferation held in Austria on Thursday and also discussed the North’s technological capabilities.

The North "would likely need several more tests to be able to make one small enough for a missile and have sufficient confidence that you can put a nuclear weapon on a warhead," he speculated.

Hecker visited the previously secret uranium enrichment facility in Yongbyon at the invitation of North Korea in 2010.

The North's "nuclear arms still are probably primitive" and its technology is not yet developed enough to mount nuclear weapons atop an intercontinental ballistic missile, he said.

The most serious possible threat would be delivery of a nuclear bomb by boat. "That would be the simplest delivery mechanism. However, it is very difficult to pull that off," he added.

