Updated at 12:39 p.m.
The United Kingdom raised its terror threat level from "substantial" to "severe" Friday, at a time when Britons have traveled to Iraq and Syria to fight alongside the Islamist militant group wreaking havoc there.
Britain’s Home Secretary Theresa May first made the announcement, but cited no specific threat in doing so. Prime Minister David Cameron later said he agreed with the decision to raise the threat level in the wake of Briton's fighting for the militant group Islamic State of Iraq and Greater Syria (ISIS). The new designation warns that a terrorist attack is "highly likely."
"I understand and I agree with the assessment that they've made," Cameron said, referring to MI5's Joint Terrorism Analysis Center, which determines the threat level independent of the Prime Minister. "That there is a greater threat that we face from Syria and Iraq, that there is a greater problem of returning foreign fighters and also it's worth remembering... you're dealing not just with [ISIS], you're also dealing with other al-Qaeda-linked franchises in Syria and indeed, potentially in Iraq."
Cameron told Britons to "continue to go about our daily lives in our normal way." He added that the changes will help the police put in place necessary security precautions.
"We must use all the resources we have at our disposal—aid, diplomacy, political influence and our military," Cameron said, adding that the U.K. supports the U.S. airstrikes against ISIS. "Learning the lessons from the past doesn't mean that there isn't a place for our military," he said.
London's Metropolitan Police commissioner Sir Bernard Hogan-Howe said earlier this week that at least 500 Britons have fought in the Iraq and Syria conflict on behalf of ISIS, and that about half have already returned to the U.K., BBC reports.
May, the Home Secretary, said the change in threat level doesn't imply, and that there is "no intelligence to suggest," that an attack is "imminent."
White House Press Secretary Josh Earnest said Friday that senior Administration officials have been in touch with their British counterparts about the change in the U.K. international terrorism threat level. "I don't anticipate at this point that there's a plan to change that level" in the U.S., added Earnest.
Department of Homeland Security Secretary Jeh Johnson confirmed in a public statement released Friday afternoon that the DHS and Federal Bureau of Investigation are "unaware of any specific, credible threat to the U.S. homeland from [ISIS]." Johnson also underscored recent efforts designed to improve U.S. national security.
"Additionally, in response to recent threats generally from overseas, the Department of Homeland Security over the past several weeks 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," said Johnson. "This government, in close collaboration with our international partners, has also taken a series of steps to track foreign fighters who travel in and out of Syria, and we are contemplating additional security measures concerning foreign fighters. Some of the security measures will be visible to the public and some understandably will be unseen."