WASHINGTON — America’s fast-growing ranks of secret cyberwarriors have in recent years blown up nuclear centrifuges in Iran and turned to computer code and electronic warfare to sabotage North Korea’s missile launches, with mixed results.

But since they began training their arsenal of cyberweapons on a more elusive target, internet use by the Islamic State, the results have been a consistent disappointment, American officials say. The effectiveness of the nation’s arsenal of cyberweapons hit its limits, they have discovered, against an enemy that exploits the internet largely to recruit, spread propaganda and use encrypted communications, all of which can be quickly reconstituted after American “mission teams” freeze their computers or manipulate their data.

It has been more than a year since the Pentagon announced that it was opening a new line of combat against the Islamic State, directing Cyber Command, then six years old, to mount computer-network attacks. The mission was clear: Disrupt the ability of the Islamic State to spread its message, attract new adherents, pay fighters and circulate orders from commanders.

But in the aftermath of the recent attacks in Britain and Iran claimed by the Islamic State, it has become clear that recruitment efforts and communications hubs reappear almost as quickly as they are torn down. This is prompting officials to rethink how cyberwarfare techniques, first designed for fixed targets like nuclear facilities, must be refashioned to fight terrorist groups that are becoming more adept at turning the web into a weapon.