on ABC's "This week." Global terror alert. The UK sounding the alarm. Increase the threat level to severe. Announcer: Warning an attack is highly likely. What has authorities so worried. Plus, more... See More
on ABC's "This week." Global terror alert. The UK sounding the alarm. Increase the threat level to severe. Announcer: Warning an attack is highly likely. What has authorities so worried. Plus, more Americans signing up to join the jihadist army. Who are these groups? And why are they now targeting teenage girls? Then, are you ready for some football? The NFL commissioner admits a mistake. Does the league's new domestic violence policy go far enough? And we're teaming up with Facebook, tracking the stories you're turning into trends. Time to reveal our first Facebook find of the week. From ABC news, "This week" with George stephanopoulos begins now. Good morning. I'm Martha Raddatz. There are fast-moving developments in the battle against Isis. Including a breakthrough for the Iraqi army against the terror group. As the U.S. Launches another round of air strikes and humanitarian drops. Plus, new fears, just 48 hours after the UK warned an attack there is highly likely. We're covering every angle. Let's start with Pierre Thomas. Reporter: Good morning, Martha. Threat level, severe. Likelihood of a terror attack? High. That's the blunt british assessment. Signs of stepped up police patrols. Hard evidence that the islamic radicals are a direct threat to the UK, the U.S., indeed all of the west. This is not some foreign conflict thousands of miles from home that we can hope to ignore. Reporter: An estimated 12,000 foreign fighters have gone to Syria. More than 100 from America. But 1,000 from Europe. You're not dealing just with isil. You're dealing with other Al qaeda-linked franchises in Syria and potentially Iraq. That's the reason for the threat level change. Reporter: American intelligence sources say they have identified no specific plot. But british authorities are likely responding to chatter from inside Iraq. Radicals whose travel documents give them easy access to Europe and the United States. They're the most urgent threat. Just in recent months, an American blowing himself up in Syria. Another dying on the battlefield fighting alongside Isis. A brit beheading American journalist James Foley. The syria-based threat that top U.S. Officials have been warning about for months now tangible. Homeland security secretary jay Johnson. Syria has become a matter of homeland security. Reporter: The FBI director? They're coming back. They're coming back to Europe. To north America. Reporter: Attorney general Eric holder. It's more frightening than anything I think I have seen as attorney general. Reporter: We caught up with holder in London as he was sounding the alarm with his european counterparts. How concerned should Americans at home be about Isis? It's just a matter of time before they start looking outward. And start looking at the west. At the United States in particular. So this is something that we have to get on top of and get on top of now. Reporter: Those words are now more urgent by the day. And Martha, remember, you have two distinct threats flowing out of Syria. Al qaeda-affiliated people that want a bomb on a commercial plane. And Isis barbarians who appear capable of anything. Thank you, Pierre. Now let's go to London. The new terror warnings have a city on edge. Jeffrey Kofman is there for us. Jeffrey? Reporter: It's been called fortress London. Police are deployed to defend the british capital against a terrorist strike. The problem here in the UK is this. What to do if some of the world's most extreme terrorists carry british passports. As Pierre just noted, the UK estimates 500 young british muslims are estimated to have joined Isis in Iraq and Syria. The fear is those militants will come home to the UK to inflict terror here. It's not an abstract concept. Britains well remember what they call 7/7. The July 7, 2005, suicide attacks that saw four british muslims detonate bombs simultaneously on three underground trains and a bus killing 52 civilians and injuring 700. In the London paper today, a fatwa was declared, forbidding british muslims from joining extremist groups. Trorm, the british parliament reconvenes. They plan to rule on measures to block british Jihadis from return LG home. Martha?
This transcript has been automatically generated and may not be 100% accurate.