WASHINGTON (AFP) - Support for the Islamic State in Iraq and Syria (ISIS) jihadist movement has grown since the US launched airstrikes in Iraq and the group is attracting many new fighters, top US officials said on Wednesday.

The ISIS group's "widespread use of social media and growing online support intensified following the commencement of US airstrikes in Iraq", FBI head James Comey told the House Homeland Security Committee.

The number of jihadist fighters in Syria and Iraq is now between 20,000 and 31,000, said Matthew Olsen, who leads the National Counterterrorism Centre.

Olsen said the ISIS group had "very sophisticated propaganda" which "exceeds" that from other groups.

"It's likely to have a potential impact on recruits," he said.

After the recent beheadings of two American hostages and a British man, Comey said the Islamic State group and "other foreign terrorist organisations may continue to try to capture American hostages in an attempt to force the US government and people into making concessions that would only strengthen (ISIS) and further its terrorist operations".

But intelligence services had no information the Islamic State group was plotting an attack inside the United States, Olsen said.

Since August 8, American forces have launched 167 raids against jihadist targets in Iraq.