A top U.S. physicist says it will probably take longer than five years for North Korea to be able to deliver a nuclear warhead long range on an intercontinental ballistic missile.(ICBM)

Dr. Jonathan McDowell of the Harvard-Smithsonian Center for Astrophysics told Voice of America Thursday, North Korea will likely possess an ICBM in two to three years. However, he said mounting a usable warhead on such a missile will take two to three years longer than that.

McDowell said previous failed test launches of the North's Musudan missiles may have been intentional explosions, as engineers only need the first 20 seconds of flight to observe trajectory in a missile's early development. 

He said two North Korean satellites launched over the past four years are orbiting the Earth, but has seen no evidence that they are actually operating.

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