The United Kingdom has raised its terror threat level to "severe" in response to the advances of the Islamic State, Prime Minister David Cameron announced on Friday.
Cameron, in a televised statement from London, said it was the first time in three years that the nation's threat level had been that high. It had previously been at "substantial."
A "severe" threat level means an attack is "highly likely," Cameron said, although he did not cite specific intelligence.
The change was made by Britain's Joint Terrorism Assessment Centre following ISIL's advance in Iraq and Syria and its videotaped beheading of American photojournalist James Foley earlier this month. Cameron noted the British accent of Foley's killer in the video, and he said "at least 500" people have now traveled from Britain to fight with ISIL in Iraq and Syria.
ISIL, Cameron said, presents "a greater and deeper threat to our security than we have known before."
He called the Islamic State "a poisonous ideology of Islamist extremism" that must be confronted head-on.
"We cannot appease this ideology. We have to confront it at home and abroad," Cameron said.
He said the country would impose tighter restrictions on the travel of foreign nationals, among other steps. He voiced support for the airstrikes President Obama has ordered in Iraq, but he gave no indication that the U.K. would be joining the military campaign.
"The key point is that military force is just one element of what we can do," Cameron said.
He said he believed the West would be fighting the extremist ideology of ISIL "for years and probably for decades"