An FBI chief has described Twitter as being the main channel for Islamic State (ISIL) recruitment saying the social media site amounted to the “devil on their shoulder” for its supporters worldwide.

Director James Comey’s social media warning came during congressional testimony about the dangers of encryption technologies that prevent law enforcement from accessing data on Americans’ smartphones.

“ISIL is reaching out, primarily through Twitter, to about 21,000 English-language followers,” Mr Comey told the Senate Judiciary Committee.

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“It buzzes in their pocket … a device, almost a devil on their shoulder, all day long saying, ‘kill, kill, kill, kill’.”

Mr Comey said ISIL’s recruitment techniques differed from al Qaeda’s, which invested more heavily in spectacular attacks against Western landmarks, ForeignPolicy.com reported.

“The Islamic State’s message is two-pronged; come to the so-called caliphate … and if you can’t come, kill somebody where you are,” he said.

“I cannot see me stopping these indefinitely.”

Silicon Valley and US law enforcement agencies had recently been at loggerheads about new versions of smartphone operating systems from Google, Apple, and other firms, that offered default encryption the companies themselves couldn’t break, ForeignPolicy.com reported.

“We have on a new scale seen mainstream products and services designed in a way that gives users sole control over access to their data,” Mr Comey said.

“As a result, law enforcement is sometimes unable to recover the content of electronic communications from the technology provider even in response to a court order or duly-authorised warrant issued by a federal judge.”

Senator John Cornyn called firms such as Apple and Google “irresponsible” for purposefully designing software that detered authorities from obtaining Americans’ phone data.