The United Kingdom raised its terror threat level to "severe" on Friday in response to events in Iraq and Syria.
"What we're facing in Iraq now with ISIL is a deeper and greater threat to our security than we have known before," U.K. Prime Minister David Cameron said in a news conference from 10 Downing Street, referring to the Islamic State by its former name, the Islamic State of Iraq and the Levant.
"With ISIL, we are facing a terrorist organization not being hosted in a country but actually seeking to establish and violently expand its own terrorist state," he added.
There is no specific threat or information suggesting that an attack is imminent, Home Secretary Theresa May said, however, the"severe" threat level indicates a terrorist attack is "highly likely."
Severe is the second-highest of five possible threat levels. The U.S. threat level remains elevated.
"This is not some foreign conflict thousands of miles from home that we can hope to ignore," Cameron said. It is a "threat to our own security in the U.K."
Britain has grown increasingly worried about the number of British citizens becoming radicalized with the intention of returning home to carry out attacks.
Speaking about the "poisonous ideology of Islamic extremism," Cameron said that at least 500 people have traveled from Britain to Syria and Iraq to fight with militant groups.
"We must take whatever action is necessary to keep British residents safe here at home," he said, adding that the U.K. has already taken steps to stop suspects from traveling by seizing passports and depriving others of citizenship.
Further steps are needed, Cameron said, explaining that he would be working on new legislation that prevents people from traveling to fight for extremist groups and makes it easier to "tackle that ideology of Islamic extremism head-on."
The Joint Terrorism Analysis Center sets the national threat level based on the latest intelligence. The threat level for the U.K. last changed in July 2011, when it was reduced to "substantial."