The British government today raised the threat level for international terrorism to "severe," indicating a terrorist attack is "highly likely" but, based on intelligence, not necessarily "imminent."
U.K. Home Secretary Theresa May released a statement Friday saying the increased threat is "related to events in Syria and Iraq where terrorist groups are planning attacks against the West, and that some of those attacks are likely to involve fighters who have travelled from the U.K."
British Prime Minister David Cameron said it was the first time in three years the threat has been raised to such a level in the U.K.
"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 in an address to the nation, noting that the terrorist group, also known as ISIS for the Islamic State of Iraq and Syria, has recruited hundreds of British citizens to its ranks.
Western security officials have been concerned for months that ISIS recruits with Western passports could travel to Syria and Iraq and then return home with deadly skills to wreak havoc.
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Cameron said ISIS's "ambition" to create an Islamic caliphate centered in Iraq and Syria - and potentially growing to the banks of the Mediterranean - "is a threat to our own security here in the U.K."
The murder of American journalist James Foley in Syria at the hands of what sounded like a British terrorist, Cameron said, was "clear evidence... that this is not some foreign conflict thousands of miles from home that we can hope to ignore."
"The links between what happens oversees and what happens here... has always been there," Cameron said.
The "severe" threat level is the second highest in the U.K.'s threat level system, topped only by "critical." The "critical" level indicates that an attack is "expected imminently."
Shortly after Cameron's comments, U.S. Department of Homeland Security Jeh Johnson released a statement saying DHS and the FBI are "unaware of any specific, credible threat to the U.S. homeland from ISIL [ISIS]."
"Plainly, however, violent extremists who support ISIL have demonstrated the intent and capability to target American citizens overseas, and ISIL continues an active and serious threat within the region," Johnson said.
White House Press Secretary Josh Earnest told reporters today he didn't anticipate that "at this point... there is a plan to change" America's current terrorism threat level.
ABC News' Josh Margolin and Aaron Katersky contributed to this report.
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Britain Raises Terror Threat Level to 'Severe'