Katharine Lackey

USA TODAY

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 greater and deeper 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 then violently expand its own terrorist state," he added.

There is no specific threat or information suggesting that an attack is imminent; however, the "severe" threat level indicates a terrorist attack is "highly likely." Severe is the second-highest of five possible threat levels.

"This is not some foreign conflict thousands of miles from home that we can hope to ignore," Cameron said. "The ambition to create an extremist caliphate in the heart of Iraq and Syria is a threat to our own security here in the U.K."

In the U.S., there were no current terror alerts on the National Terrorism Advisory System, according to its website.

"I don't anticipate at this point that there is a plan to change that level," White House press secretary Josh Earnest said at a press briefing Friday.

U.S. intelligence shows "no evidence or indication right now that ISIL is actively plotting to attack the U.S. homeland," he added.

Earnest said that the threat to the U.S. is "not dissimilar" to that faced by the U.K., but he also noted that there are "a relatively large number of individuals with British passports who have gone to the region," while the number of Americans who are there is "somewhat lower."

"For a number of months now, we have been monitoring those individuals that have Western passports, that are citizens of Western countries, either United States or in Europe, who have made the decision to travel to Syria or take up arms alongside ISIL," Earnest said. "They pose a threat because they've received military training, they are now battle-hardened, and they've demonstrated a willingness to risk their lives for their cause."

The Department of Homeland Security and FBI are not aware of any specific, credible threats to the USA from the Islamic State, Homeland Secretary Jeh Johnson said in a statement.

"At the same time, the Department of Homeland Security is mindful of the potential for homegrown violent extremism inspired by radical ideology overseas, and, both through law enforcement and community outreach, we are taking steps to counter that potential threat," Johnson said.

A number of steps have been taken to enhance aviation security at overseas airports in recent weeks, as well as track foreign fighters who travel overseas, Johnson said.

"We are contemplating additional security measures concerning foreign fighters," he added.

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 the U.K. to Syria and Iraq to fight with militant groups.

"We will always take whatever actions necessary to keep the British people safe here at home," he said, adding that the U.K. has already taken steps to stop suspects from traveling by seizing passports, barring foreign nationals from re-entering the country and depriving others of citizenship.

Further steps are needed, Cameron said, explaining that he would be working on new legislation that makes it easier to take people's passports away.

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."

The last time the threat level reached "critical," the highest level, was in late June 2007, after a car on fire was driven into Glasgow Airport and bombs were discovered in vehicles in London.

Contributing: Gregory Korte in Washington, D.C.