LONDON (AP) -- British counterterrorism forces are monitoring "thousands" of potential extremists in the capital's metropolitan area, London Mayor Boris Johnson said in a newspaper interview published Saturday, as police continue to question five men arrested on suspicion of terrorist offenses.
His comments came as police chiefs warned police officers throughout Britain to use extra care because of an increased threat to their personal safety
The Daily Telegraph newspaper quoted Johnson as saying the security forces in the London area are involved in "thousands of operations" every day. He said the number of people being watched is in the low thousands.
"In London we're very, very vigilant and very, very concerned," he said, adding that the national threat level had recently been raised. It is now listed as "severe," meaning an attack is viewed as highly likely.
Johnson's comments suggested the number of potential extremists is higher than had been previously believed. He said many of the estimated 500 fighters who had gone from Britain to Syria to support Islamic State group militants there had come from the greater London area - posing a security threat if they return after having received training in explosives and weapons use.
Assistant Metropolitan Police Commissioner Mark Rowley, a director of counterterrorism efforts, said the new warning to police officers about their safety was made because "the threat level to police officers and staff has been raised."
He said police are being asked to "remain vigilant and alert to any possible dangers."
Rowley didn't specify whether the increased threat to police is related to the arrests of the five men earlier this week.
Government officials, speaking on condition of anonymity because they weren't authorized to discuss the case with the media, have said the men, aged 20 and 21, include one who recently returned from Syria. The officials said police are probing possible ties to the Islamic State group operating there.
The men, including one medical student, can be held for questioning until Oct. 14. That period could be extended under terrorism laws.