NEW YORK (REUTERS) - Islamic State in Iraq and Syria (ISIS) has"revolutionised" terrorism by seeking to inspire small-scale individual attacks around the world through social media, encrypted communications and slickly produced propaganda, FBI director James Comey said on Wednesday.

"Your parents' Al-Qaeda was a very different model than the threat we face today," Comey said at a counterterrorism conference in New York City.

He said the Federal Bureau of Investigation currently has "hundreds" of investigations in all 50 US states involving potential ISIS-inspired plots.

His remarks came as Americans are jittery two weeks after a married couple shot and killed 14 people in San Bernardino, California, in an attack inspired by ISIS.

The group is based in Iraq and Syria, where it controls a large area of territory as it seeks to carve out a caliphate. It claimed responsibility for attacks in Paris on Nov 13 that killed 130 people.

Comey said ISIS has perfected the use of social media, and Twitter in particular, to contact potential followers in the United States and elsewhere.

"Twitter works as a way to sell books, as a way to promote movies, and it works as a way to crowdsource terrorism - to sell murder," Comey said.

ISIS also employs "end-to-end" encryption when communicating with individuals who it believes are willing to carry out killings in its name, Comey said.

That has posed a significant challenge for investigators, who often find themselves stymied even when they have court orders giving them access to devices.

Comey said he is convinced that law enforcement and technology companies can work together to solve that problem without compromising personal privacy.

"We are not going to break the Internet," he said.

"We are not going to jeopardise people's security."