Up to 20 sleeper cells comprising 120 to 180 people may be ready to strike targets in several European countries, intelligence source say in the wake of the discovery of an "imminent threat" of an attack in Belgium this week.

More than two dozen people with suspected ties to Islamic extremists were arrested in a sweep Friday,  and an unnamed Western intelligence source told CNN that as many as 20 sleeper cells are ready to strike targets in France, Germany, Belgium, and the Netherlands.

"There is a tremendous amount of concern over sleeper cells in Europe," said the official, who CNN reports has direct knowledge of the evolving situation.

Authorities are concerned about copycat attacks through Europe following the incidents that unfolded in Paris earlier this month, said the official, comparing the terror threat to a "slow-motion car accident happening right in front of us."

Intelligence officers have been monitoring men who have returned to Europe after fighting in Syria, as it is believed the Islamic State group, also known as ISIS, has directed attacks to retaliate against airstrikes against its forces in Syria and Iraq, a senior European counterterrorism official said.

The United Kingdom, Belgium, the Netherlands, and France are all part of a U.S.-led coalition conducting airstrikes against ISIS in Iraq.

On Friday, 17 suspected terrorists were arrested, with 13 captured in Belgium and the other four in France.

Meanwhile, it's suspected that the accused Islamist terrorists involved in Thursday's shootout with police in Belgium have links to terror cells throughout Europe, according to a senior counterterrorism source in Belgium. Further, two of the suspects killed are believed to have part of the ISIS forces in Syria.News Update

But despite the threat, the Netherlands said Friday it will not raise its terror threat level, which is at "substantial," the second-highest level.

"That means there is a realistic threat, but no concrete or specific information of an attack," said government spokesman Edmond Messchaert Friday.

In Belgium, the current security level will remain for at least one month and will be evaluated to see if it needs to be modified, Prime Minister Charles Michel told CNN affiliate RTL.

"From the time we are confronted with an increase in the threat, we went to level 3 on a scale of 4; we must mobilize the resources available," Michel told CNN.

Counterterrorism experts are still tracking people associated with brothers Said and Cherif

Kouachi and another man Amedy Coulibaly, who were killed after the Paris attacks.

Neetin Karasular, an accused Belgian weapons trafficker who reportedly met with Coulibaly's widow, Hayat Boumeddiene, has been arrested, but his attorney claims his client was not picked up as part of the raids in Belgium.

Karasular was charged with association with wrongdoers and firearm offenses, in Charleroi, Belgium. But his attorney said the arrest was not connected to the other Belgian raids