British Prime Minister David Cameron is warning that the threat to the U.K. from international terrorism is "greater and deeper" than ever before, as London raised its terror warning level in response to what it said were plans by the Islamic State and other extremist groups to attack the West.
The U.K. ratcheted up the alert to "severe," which is the second-highest of five levels and indicates the risk of an attack is considered "highly likely." However, it's important to note that authorities have not offered evidence of a specific or immediate plot. The level has been higher in the past — at "critical" in 2006 and again in 2007.
Ahead of Cameron's remarks, Theresa May of the U.K.'s Joint Terrorism Analysis Center said some plots "are likely to involve foreign fighters who have traveled there from the U.K. and Europe to take part in those conflicts."
The prime minister expressed particular concern about what he said were at least 500 people who had traveled from the U.K. "to fight in Syria and potentially Iraq." British authorities have been trying to track down the assassin of American journalist James Foley, who spoke with a British accent on the videotape released by his Islamic State captors.
Professor Andrew Silke of the University of East London told the BBC that he believes today's announcement is a reaction to the killing of Foley.
Cameron said the Islamic State militant group, also known as ISIL or ISIS, poses a "greater and deeper threat to our security than we have known before."
"We are in the middle of a generational struggle," Cameron said, adding that it could take years. "The scale of this threat is growing."
He said there is "no doubt" that the Islamic State is targeting "all of us in Western Europe."
"Britain is an open, tolerant and free nation," but it will take steps to stop people traveling by seizing their passports if they're deemed a threat, Cameron said.
The prime minister added that it is important to make a distinction between religion and political ideology. The religion of Islam, he said, "inspires millions to countless acts of kindness," but Islamic extremism is a political ideology that "is in the minority.
"We cannot appease this ideology," Cameron said.
The BBC says the threat level went to "critical" in August 2006, after police uncovered a plot to blow up passenger planes, and again in June 2007 after an attack at Glasgow airport. The last time it was changed was in 2011, when it was lowered to "substantial."
Update at 1:00 p.m. ET. Homeland Security: No 'Specific, Credible' Threat To U.S.
Homeland Security Secretary Jeh Johnson said in a statement after the British announcement:
"At present, the U.S. Department of Homeland Security and the FBI are unaware of any specific, credible threat to the U.S. homeland from ISIL [Islamic State]. Plainly, however, violent extremists who support ISIL have demonstrated the intent and capability to target American citizens overseas, and ISIL constitutes an active and serious threat within the region."