The new mobile missile, called the KN-08, has not yet been operationally deployed, and American officials say it may not be ready for some time. But the discovery that the mobile units have already been dispersed around the country, where they can be easily hidden, has prompted the White House, the Pentagon and intelligence agencies to reassess whether North Korea’s missile capabilities are improving at a pace that poses a new challenge to American defenses.

On Thursday, speaking in Italy, the departing defense secretary, Leon E. Panetta, broke from the usual Obama administration script — which is to write off North Korea as a broke and desperate country — and told American troops that he was increasingly worried about another, longer-range North Korean missile, one that was successfully tested last month and reached as far as the Philippines, and could lob a warhead much farther.

“Who the hell knows what they’re going to do from day to day?” Mr. Panetta said.