(CNN) -- The UK government raised its terror threat level Friday from substantial to severe, the fourth highest of five levels, in response to events in Iraq and Syria, where ISIS militants have seized a swath of territory.
"That means that a terrorist attack is highly likely, but there is no intelligence to suggest that an attack is imminent," Home Secretary Theresa May said.
"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."
UK authorities estimate that about 500 Britons have gone to Syria and Iraq to fight with Islamist groups.