WASHINGTON (AFP) - Senior US national security officials on Thursday (July 14) warned about a possible rise in extremist violence with the spread of a "terrorist diaspora" once the Islamic State group in Iraq and Syria (ISIS) is defeated.

"We all know there will be a terrorist diaspora out of the caliphate as military forces crush the caliphate," FBI director James Comey told a hearing of the House Homeland Security Committee, referring to the Islamic state proclaimed by the militant group.

Thousands of fighters will spread worldwide "and our job is to spot them and stop them before they come to the United States to harm innocent people," he added.

The ISIS group's defeat in Iraq and Syria will make it "desperate to demonstrate its continued vitality, and that is likely to take the form of more asymmetric attacks, of more efforts at terrorism," Comey said.

CIA director John Brennan estimated last month that some 18,000 to 22,000 ISIS fighters remained in Iraq and Syria, and he predicted they would probably intensify attacks around the world even as they come under pressure in Iraq and Syria.

The Islamic militant group has ceded an increasing amount of territory in recent weeks, especially in Iraq, where it lost control over the Sunni city of Fallujah and a major air base in Qayyarah, some 60km from the city of Mosul, the ISIS group's de facto capital in Iraq.

However, that has not stopped the militants from staging deadly attacks in Baghdad and elsewhere.

Although the ISIS group's grip in Iraq and Syria is weakening, there may be a significant lag between the US-led coalition's battlefield victories and reducing the ISIS group's capability to stage attacks, Nicholas Rasmussen, director of the National Counterterrorism Centre, told the committee.

Brennan said on Wednesday that three suicide bombings in Saudi Arabia last week bear the hallmarks of ISIS.