The U.K. in response to a threat posed by the Islamic State of Iraq and Syria raised its terrorist attack warning system to the second highest level Friday from substantial to severe, which carries the warning that a terrorist attack is "highly likely."

British Prime Minister David Cameron said British citizens should continue to go about their daily lives in a normal way.

Ratcheting up the threat level, he said, “does trigger a series of actions to make sure that they’ve deployed all the extra security that you can … so you might see an increase in police” on the streets.

The U.S. National Terrorism Advisory System’s website shows no current threat, the lowest level. The system has three threat levels: none; elevated, which means a credible terrorist threat; and imminent, which means a credible, specific and impending threat.

In April 2011, then-Homeland Security Secretary Janet Napolitano replaced the original color-coded terrorist threat alert system with the new version.

White House spokesman Josh Earnest Monday said there is no active ISIS plot to attack the U.S. homeland and cited a U.S. intelligence assessment.

“There is no evidence of an active plot right now,” he said, although he noted that ISIS poses a definite threat to the United States and demonstrated last week their willingness to “perpetrate terrible acts” against the United States.

Earnest was referring to ISIS’ circulation of a video showing the beheading of American journalist James Foley.