In United States, the FBI has arrested more than half a dozen individuals seeking to travel from US to Syria to join the fighting there, possibly with ISIS.

However, more needs to be done to gain intelligence on these foreign fighters.

Testifying before the Committee on Homeland Security, on ‘Worldwide Threats to the Homeland’, the FBI Director James Comey expressed concern over home grown terrorists.

"These are the home-grown violent extremists that we are sorry about, who can get all the poison they need and the training they need to kill Americans, and in a way that's very hard for us to spot. Those are the two ways in which I've seen the terrorism threat change significantly since I was last in government," he said.

The National Counter Terrorism Centre Director, Matt Olson, said by every measure, ISIL has emerged as an extremely dangerous organization in a very chaotic part of the world.

The group has exploited the civil war in Syria, taken advantage of sectarian tensions in Iraq to entrench itself in both countries. It has also established sanctuaries in Iraq and in Syria.

From where the group has the ability to plan and to train and also to amass both fighters and weapons with really little interference, he said.

"The group's proven to be an effective fighting force. Its battlefield strategy is complex and it's adaptive. It uses a mix of techniques from terrorist operations to hit-and-run tactics, to paramilitary assaults to enable their recent gains," Olson said.

These radicalized Westerners represent an exceptionally grave threat to US homeland because of their militant training, extremist connections, ease of travel, and intimate knowledge of the West, said Congressman Michael McCaul, chairman of Committee on Homeland Security.

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