A Western intelligence source revealed that there could be as many as 20 sleeper cells of between 120 and 180 people ready to strike in France, Germany, Belgium and the Netherlands

A Western intelligence source revealed that there could be as many as 20 sleeper cells of between 120 and 180 people ready to strike in France, Germany, Belgium and the Netherlands.

The source told CNN national correspondent Deborah Feyerick that European Union and Middle East intelligence agencies had identified an "imminent threat" to Belgium, and possibly also to the Netherlands.

The warning comes against the backdrop of the deadly attacks in Paris last week and anti-terror raids in Belgium on Thursday that have European counterterrorism agencies scrambling to identify and thwart potential threats.

European security services in recent weeks have received indications that the extremist group ISIL may have started directing European adherents in Syria and Iraq to launch attacks in their home countries, a senior European counterterrorism official told CNN analyst Paul Cruickshank.

The official said security agencies in a number of European countries were investigating groups of returnees from Syria and Iraq, including people back on Belgian soil, who they suspect could be plotting terrorist attacks.

Belgian authorities said Thursday's raids, in which two suspects were killed, were part of an operation investigating a cell that included people returning from Syria and that was about to carry out major terrorist attacks in Belgium.