LONDON - The United Kingdom Friday raised the terror threat level to "severe" in the wake of a Sunni Islamist terror group rapidly expanding its influence and control in parts of Iraq and Syria.
British Prime Minister David Cameron told reporters at a news conference from 10 Downing Street that the "root cause" of the terrorist threat in the United Kingdom was "Islamist extremism".
He referred to the killing of American journalist James Foley in sighting as "clear evidence" that militant group Islamic State's fight in Iraq and Syria "is not some foreign conflict thousands of miles from home that we can hope to ignore".
"What we're facing in Iraq now with ISIL (Islamic State in Iraq and Levant) is a deeper and greater threat to our security than we have known before," said Cameron, referring to the Islamic State by its former name, the Islamic State of Iraq and the Levant.
"With ISIL we are facing a terrorist organization not being hosted in a country but actually seeking to establish and violently expand its own terrorist state," he added.
He addressed the news conference after Home Secretary Theresa May said a terrorist strike in the United Kingdom was "highly likely" even as there was no intelligence of an imminent attack.
"That means that a terrorist attack is highly likely, but there is no intelligence to suggest that an attack is imminent," May said.
The terror threat level "severe" is the second-highest of five possible threat levels.
"The increase in the threat level is related to developments in Syria and Iraq where terrorist groups are planning attacks against the West. Some of those plots are likely to involve foreign fighters who have traveled there from the UK and Europe to take part in those conflicts."
May urged Britons to stay alert.
"I would urge the public to remain vigilant and to report any suspicious activity to the police," May said.
Authorities in the United Kingdom estimate that about 500 Britons have gone to Syria and Iraq to fight with Islamist groups.
According to the BBC, Britain has grown increasingly worried about the number of British citizens that have been radicalized, mostly online, by the extremist Islamist ideology.
Intelligence officials, speaking privately, said the authorities were worried that Britons who have joined the Islamic State may return home with the intention to carry out attacks.
Cameron at the press conference spoke about "poisonous ideology of Islamic extremism" and said: "We must take whatever action is necessary to keep British residents safe here at home."
Further steps are needed, Cameron said, to "tackle that ideology of Islamic extremism head on".