The UK government raised its terror threat level Friday from `` substantial '' to `` severe , '' the fourth highest of five levels , in response to events in Iraq and Syria , where ISIS militants have seized a large swath of territory . `` That means that a terrorist attack is highly likely , but there is no intelligence to suggest that an attack is imminent , '' Home Secretary Theresa May said . The `` root cause '' of Britain 's terror threat is `` Islamist extremism , '' Prime Minister David Cameron said . The execution of American journalist James Foley is clear evidence that ISIS 's fight in Iraq and Syria `` is not some foreign conflict thousands of miles from home that we can hope to ignore , '' according to the UK leader . ISIS is unlike other Islamist extremist groups in its primary focus not to find a country that can be its base of operations , but to create its own country . And the group has had ample success in that regard , given the vast reach already of what it calls the Islamic State . While it 's been widely reviled internationally , ISIS has managed to attract some support among Muslims and drawn foreign fighters , like the masked man with an apparent British accent who took part in Foley 's beheading , who some fear could soon carry out attacks back home . Even without specific threats in the West , ISIS ' track record in Syria and Iraq -- where it was known to massacre minorities , forcefully institute Sharia law and stage executions and stonings -- suggest it may be capable of anything . Cameron said the group poses a `` greater and deeper '' threat than Britain has ever known . `` This is al Qaeda version 6.0 , '' Ryan Crocker , a former U.S. ambassador to Iraq and Syria , told CNN on Friday . `` They are like nothing we have ever seen before . '' Travel restrictions , an increase in police activity . ISIS has been operating for years ; now its actions in Iraq are prompting the United States to target its fighters with airstrikes there and to threaten more such strikes in Syria . Cameron said military force is among the tools that can be used against ISIS , while adding that aid , diplomacy and political influence should also be part of Britain 's response . That said , his focus Friday was n't so much about what to do about ISIS overseas as it was keeping British citizens back home safe . The Prime Minister vowed he will soon announce plans to stop would-be jihadists from traveling to Syria and Iraq and to make it easier to take their passports away . Britain also needs to do more to stop current fighters from returning from the Middle East and to deal decisively with those who already have returned , he said . UK authorities estimate that 500 Britons have gone to Syria and Iraq to fight with Islamist groups . Britain 's national policing lead for counterterrorism , Mark Rowley , said -- while the threat level just went up Friday -- police have been escalating their efforts to combat the jihadist threat for months . He claimed 69 arrests in the first half of 2014 for offenses ranging from funding `` terrorist activity through to the preparation and/or instigation of terrorism acts and traveling abroad for terrorist training . '' CNN National Security Analyst Peter Bergen says Britain likely hiked its terror threat level because `` there 's no way you can track 500 people . '' At the same time , he downplayed the idea that Friday 's announcement means an attack is imminent . `` This is what governments are paid to do : They are paid to worry about their citizens , '' Bergen said , noting that the UK threat level has gone up before without a subsequent attack . `` That does n't mean their citizens should be in a constant state of worry . '' Holiday security amped in U.S. -- but not due to ISIS . Security will increase this long Labor Day weekend for those traveling in the United States -- though not necessarily because of ISIS . A U.S. federal official says its customary to see increased security presence leading up to a holiday weekend , but that 's not necessarily tied to an increased terror threat . To that point , White House spokesman Josh Earnest said he does n't anticipate the U.S. terror threat level will rise anytime soon . And U.S. Secretary of Homeland Security Jeh Johnson pointed out there is no specific threat against the United States . That said , Johnson noted that ISIS has proved to be a threat to Americans overseas -- with Foley 's execution and the threats of more killings to follow being exhibit No . 1 . The Homeland Security department `` has taken a number of steps to enhance aviation security at overseas airports with direct flights to the United States , and the United Kingdom and other nations have followed with similar enhancements . '' Daniel Benjamin , a former State Department counterterrorism director in President Barack Obama 's administration , acknowledged ISIS sympathizers could lash out domestically or Americans who 've gone to fight with militants could bring that fight back home . Still , that does n't mean we should expect a coordinated attack outside the Middle East anytime soon , he said . `` I think there is a certain amount of hyperventilation at the moment , '' said Benjamin , now head of Dartmouth College 's John Sloan Dickey Center for International Understanding . `` The long-term threat is quite serious ... But this is a group that has no track record of plotting terrorist attacks outside of its own area . '' Report : ISIS computer included files on biological weaponry . So what is ISIS capable of -- whether it 's in the Middle East or elsewhere ? That 's hard to answer definitively , since the group has n't lashed out outside the Middle East like al Qaeda . A recent story in Foreign Policy magazine , however , suggest that ISIS is thinking big . The report talks about files on a laptop that purportedly once belonged to ISIS before being abandoned in Syria . They files reportedly include talk about biological and other such weapons , including the weaponizing of the bubonic plague . If these documents are legitimate , they 'd suggest that ISIS has a grand strategy and is willing to kill thousands -- including civilians -- using weapons of mass destruction to further its aims . But that does n't mean the group is close to putting such plans into action . CNN Terrorism Analyst Paul Cruickshank cautioned that there 's a big difference between researching chemical and biological agents and creating the capability to use them . He noted that al Qaeda tried for two decades to develop such weapons , with no apparent success . And there 's no evidence that ISIS has a dedicated WMD program of its own , said Cruickshank , who suggested the files might `` just be one guy , doing research . '' `` It 's extremely hard to weaponize these kind of agents -- especially on the delivery side , '' he added . Airstrikes continue . Still , even if ISIS has n't deployed or even developed chemical or biological weapons , its fighters have proven to be plenty effective with conventional weaponry . Yet now , in Iraq at least , ISIS forces find themselves under fire . The U.S. military has carried out 110 airstrikes in that country since August 8 , at an average cost of $ 7.5 million a day , said Pentagon spokesman Rear Adm. John Kirby . That includes more strikes Friday targeting ISIS fighters near the Mosul Dam , according to U.S. Central Command . The strikes destroyed four armored vehicles , damaged one more and destroyed three support vehicles , all belonging to the terrorist group , the military said . Iraq 's air force continued its own assault on ISIS-controlled Falluja , with the militant group responding in turn . The bombardment killed 18 people and wounded 57 others , a Falluja hospital official and two residents at the facility told CNN . Kirby , like the British Prime Minister , argued that military action alone is not enough to combat extremism . `` If we 've learned nothing over 13 years of war , it 's that you ca n't completely eliminate extremism anywhere through simply kinetics , through airstrikes alone , '' he said at a press briefing Friday . Obama 's no 'strategy yet ' comment on ISIS in Syria sparks a political uproar .
Ex-U.S. official notes ISIS has yet to attack outside the Middle East . UK terror threat hiked to second-highest level due to events in Iraq , Syria . Cameron : "We will not be cowed by this barbaric killing" Score: 6 Spokesman : U.S. has no plans now to raise its own terror threat level .