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 large portions of the 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.
British Prime Minister David Cameron said the "root cause" of the terrorist threat in the United Kingdom is "Islamist extremism," adding that 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."
The threat that ISIS poses UK security is "greater and deeper" than the country has ever known, Cameron said.
"With ISIL, we are facing a terrorist organization not being hosted in a country but actually seeking to establish and then violently expand its own terrorist state," he added.
According to reports, an estimated 500 Britons have gone to Syria and Iraq to fight with the terrorist group, and Britain's national policing lead for counterterrorism, Mark Rowley, revealed that police activity relating to the jihadist threat has escalated for months.
"The ongoing commitment to fight this threat has seen a significant rise in the number of Syria-related arrests," he said in a statement. "The first half of this year has seen a total of 69 arrests made for a variety of offenses, including fundraising for terrorist activity through to the preparation and/or instigation of terrorism acts and traveling abroad for terrorist training."
To counter this unnerving trend, Cameron said he will soon announce plans to stop would-be jihadists from traveling to Iraq and Syria and will make it easier to take passports away.
In the U.S., however, there were no current terror alerts on the National Terrorism Advisory System, according to its website.
"I don't anticipate at this point that there is a plan to change that level," White House Press Secretary Josh Earnest said at a press briefing Friday, adding that U.S. intelligence shows "no evidence or indication right now that ISIL is actively plotting to attack the U.S. homeland."
While concern in the U.S. is "not dissimilar" to the one in the U.K., Earnest stated that far more Britons have reportedly joined ISIS than Americans.
But while the Department of Homeland Security and FBI are not aware of any specific, credible threats to the USA from the Islamic State, Homeland Secretary Jeh Johnson noted that "The Department of Homeland Security is mindful of the potential for homegrown violent extremism inspired by radical ideology overseas, and, both through law enforcement and community outreach, we are taking steps to counter that potential threat."