Britain has raised its terror threat alert to the second highest level, signifying a terror attack is considered "highly likely."

Prime Minister David Cameron says his country faces a growing terror threat. His remarks are raising questions about President Barack Obama's response to the Islamic State in Iraq and Syria and whether the threat level in the United States should be raised.

Obama does not appear ready to commit to fighting ISIS with airstrikes in Syria, but Britain is clearly feeling the pressure that ISIS could strike at home.

"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.," Cameron says.

It was a British voice on the tape showing the murder of American journalist James Foley last week. An estimated 500 people have traveled from Britain to fight in Syria and Iraq along with hundreds of other Europeans.

U.S. and European security services believe ISIS fighters are back at several locations in Europe, but those cells may not be under direct ISIS orders. Officials have not said where the cells are located.

Officials say they do not believe there is a cell in the U.S., but are tracking about a dozen Americans fighting for ISIS overseas. Just this week, two American ISIS fighters were believed killed in Syria.

 

The Department of Homeland Security says it is "unaware of any specific, credible threat," from ISIS.