Senior U.S. officials believe, based on the latest intelligence, that North Korea is not an immediate threat to the U.S. or Japan.

But South Korea is another story. The threat to South Korea, said one official, is "grave and current."

A senior intelligence official said that the U.S. assessment is that North Korea has eight to 10 nuclear weapons, with the number more likely to be eight than 10. That number is in line with estimates by outside experts. The Federation of American Scientists, for example, estimates 10-to-20, but as Hans Kristensen, director of the FAS Nuclear Information Project, says, "it is unclear if they are operational yet."

Miniaturizing warheads to put them on long-range missiles might be a challenge for the North Koreans. But Seoul is less than 40 miles from the DMZ that separates South and North. Korea has many shorter range missiles that could theoretically deliver nuclear payloads inside South Korea, as well as to border regions of China and Russia.