British police Tuesday urged people to identify "aspiring terrorists" among their family members, friends and neighbours after the killing of US journalist James Foley, apparently by a man with an English accent.
The appeal also comes amid growing government concern that British passport holders who travel to fight in Iraq and Syria could return to carry out attacks on home soil.
Jihadist group the Islamic State (IS) posted a graphic video online last week showing the beheading of Foley, who had been missing since his 2012 capture in Syria.
"We are appealing to the public, family members and friends to help identify aspiring terrorists; they may be about to travel abroad, have just returned or be showing signs of becoming radicalised," said Scotland Yard Assistant Commissioner Mark Rowley, the country's most senior police officer on counter-terrorism, in a statement.
"Every reasonable person in the country has been touched by the pitiless murder of James Foley at the hands of Islamic State terrorists, and the murderer's apparent British nationality has focused attention on extremism in the UK as well as the Middle East."
He said British police had arrested five times more people in the first half of this year compared with 2013 for "Syria-related" offences.
There were 69 arrests in the first half of 2014 on suspicion of offences including travelling abroad for terrorist training, preparing acts of terrorism and fundraising for terrorist activity.
Some 1,100 pieces of extremist material are also being removed from social media websites such as YouTube, Facebook and Twitter, he added, 800 of them relating to Syria or Iraq.
Intelligence services say 500 Britons have travelled to Syria or Iraq to fight alongside jihadists in the last few years.
The government is under increasing pressure to take steps to combat radicalisation and Home Secretary Theresa May said Saturday that she was considering introducing new powers.
Foreign Secretary Philip Hammond on Sunday called it an "utter betrayal" that the killing had apparently been carried out by a Briton.
Rowley also said that "significant progress" was being made in the hunt for Foley's killer and Britain's ambassador to the US, Peter Westmacott, told CNN Sunday that the country's authorities were "close" to identifying the man.