The FBI warned lawmakers Wednesday there was no way to monitor encrypted online communications among Islamic State sympathizers and called for new laws to require technology firms to unlock any secret messages among jihadists.

The IS group often moves sensitive online conversations about possible attacks to an encrypted "dark space" beyond the reach of US law enforcement's surveillance methods, said Michael Steinbach, assistant director of the FBI's counter-terrorism division.

Encrypted communications has "afforded a free zone by which to recruit, radicalize, plot and plan," Steinbach told the House Homeland Security Committee.

He said the situation was "troubling" and acknowledged law enforcement agencies did not know the volume of the hidden online messages.

"We're past going dark in certain instances. We are dark," Steinbach said.

Michael McCaul, chairman of the committee, called the problem a "tremendous threat to our homeland."

The senior FBI official urged Congress to grant new authorities to law enforcement agencies that would allow them access to encrypted online communications, either stored messages in an archive or messages in real-time.

"We suggest and we are imploring Congress to help us seek legal remedies towards that," Steinbach said.

And the government is also "asking companies to help provide technological solutions" to address the threat, he added.

Civil liberties groups oppose such proposed measures but Steinbach insisted privacy rights would not be endangered by the approach.

"We're not talking about large scale surveillance techniques," Steinbach said.

US authorities would still have to seek permission from a court and prove that its request for surveillance was justified, he said.

His comments came at a hearing titled "Terrorism Gone Viral" that focused on the implications of an attack in Garland, Texas last month in which gunmen inspired by the IS group's social media tried to storm an event featuring caricatures of the Prophet Mohammed.

"This event highlights the growing threat our nation faces from a new generation of terrorists, often operating from afar, who use social media to find like-minded associates within our borders who can be motivated to violence, attacking with little or no warning," John Mulligan, deputy director of the National Counter-Terrorism Center, told lawmakers.