the UK's counter-terrorism chief has said "significant progress" is being made in the hunt for the apparently British murderer of American journalist James Foley.
Metropolitan Police Service Assistant Commissioner Mark Rowley appealed for help in rooting out aspiring home-grown terrorists as he decried Mr Foley's "pitiless" death at the hands of Islamic extremists in Iraq.
Islamic State terrorists posted a film online last week of the horrifying murder of hostage Mr Foley, 41. It appeared to show he was killed by a man who spoke with a British accent.
Mr Rowley, the UK's top counter-terrorism officer, said: "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. Investigators are making significant progress."
He appealed to family and friends of aspiring UK terrorists to come forward, stressing that nearly half of would-be jihadis identified as part of Syria-linked investigations had not previously been classed as dangerous.
In the first half of this year 69 suspected extremists who planned to travel to Syria or fund terrorism there were arrested, and five times as many suspects are being arrested now compared to 2013.
Mr Rowley said: "High priority operations, especially against those involved in attack planning or on the cusp have increased greatly. Port stops and cash seizures have grown by over 50% as we strive to disrupt terrorists.
"The biggest growth in Syrian-related investigations has occurred in London and the West Midlands. The growth of dangerous individuals poses challenges for policing, especially when nearly half of Syria travellers of concern were not known as terrorist risks previously."
Scotland Yard investigators are having hundreds of pieces of terrorist content linked to Syria removed from websites including YouTube, Facebook and Twitter.
It is estimated about 500 Britons have travelled to Syria to fight alongside extremists there, of whom around half are thought to be from London.
Meanwhile, London Mayor Boris Johnson has said he wants Mr Foley's murderer to be killed in a "bunker buster" bomb attack.
Mr Johnson also joined the growing calls for Britons fighting abroad to be stripped of their citizenship.
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