Current U.S. missile defenses can counter an attack on U.S. territory by North Korea or Iran but Washington will have to boost its response capacity if those countries keep expanding their missile forces, a top U.S. admiral said on Thursday.

Adm. Bill Gortney, the officer responsible for defending U.S. air space, told a Senate panel it was "prudent" for him to assume North Korea had the ability to miniaturize a nuclear weapon and put it on an intercontinental ballistic missile that could target the United States.

"Intel community gives it a very low probability of success, but I do not believe the American people want (me) to base my readiness assessment on a low probability," he added.