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."

(CNN)