LONDON (KABC) --Less than two weeks away from the anniversary of Sept. 11 and the terror threat level in some parts of the world has never been higher.
From the U.S. to the U.K., world leaders are warning of an imminent and dire threat from Islamic militant groups like ISIS.
British Prime Minister David Cameron announced Friday that the U.K. has raised it's terror threat level from substantial to severe.
"What we're facing in Iraq now with ISIL is a greater and deeper threat to our security than we have known before," Cameron said.
British nationals fighting for ISIS, or ISIL, are considered some of the group's most brutal fighters. U.S. officials confirmed Friday that a team of Brits was behind last week's execution of American journalist James Foley, including the killer, believed to be from London. Worries continue about more Americans heading overseas to join terrorist ranks.
"We have the prerogative at the state department in coordination with law enforcement authorities to revoke passports when it comes to a point where somebody is working with a terrorist organization or posing a threat to the American public," State Department spokeswoman Jen Psaki said.
Two U.S. citizens were killed fighting for ISIS in Syria this week, and then there's the case of Moner Mohammad Abu-Salha. He grew up in Florida and was able to fly to and from Syria, train as a jihadist, then in May, blew himself up in a suicide attack in Syria.
"If we know something about somebody, they are going to be questioned," Department of Homeland Security Former Assistant Secretary Stewart Verdery said. "The real concern is is the sleeper cell, somebody who has kept under the radar."
As for fighting ISIS abroad, the White House looked to explain President Barack Obama's comments Thursday on his plan to deal with ISIS in Syria.
"I don't want to put the cart before the horse. We don't have a strategy yet," he said.
"The president hasn't yet laid out a specific plan for military action in Syria, and the reason for that is simply that the Pentagon is still developing that plan," White House Press Secretary Josh Earnest said.