U.K. security services are closely watching “thousands” of terrorist suspects in London, city Mayor Boris Johnson said in remarks published by the Telegraph Friday. The statement came after two Britons were recently killed by the Islamic State group, formerly known as ISIS.

The U.K. raised its international threat level to the second-highest classification of “severe” in August. British Prime Minister David Cameron later said the threat is highest in association with fighters belonging to the Islamic State group, currently engaged in Iraq and Syria.

Johnson said the threat to London is comparatively more severe in connection with home-grown terrorist plots, as between 500 and 600 people have traveled from the U.K. to join ISIS or al Qaeda -- and they may be returning.

“In London, we’re very, very vigilant and very, very concerned. Every day -- as you saw recently, we had to raise the threat level -- every day, the security services are involved in thousands of operations. There are probably in the low thousands of people that we are monitoring in London,” Johnson told the Telegraph.

Johnson expressed concern that between one-third and one-half of those who have traveled from the U.K. to join militant groups in the Middle East formerly lived in his city: Authorities will have a “real job to deal with them” whenever they return, he said.

Meanwhile, five men were arrested by police Tuesday in conjunction with what was described as an operation against Islamist-related terrorism, Reuters reported.

Johnson’s comments were published the day after Britain’s most senior counter-terrorism officer, Mark Rowley, warned police around the country to be alert about possible risks to their safety, saying, “Measures are being put in place to increase the vigilance of officers and staff.”