As many as twenty sleeper cells are operating throughout Europe, an intelligence source told CNN on Friday. The cells, made up of between 120 to 180 people, were reportedly planning attacks in France, Germany, Belgium and the Netherlands. The source said that intelligence agencies have identified an imminent threat to Belgium, causing Jewish schools in the country to close on Friday. Belgian police are holding 13 suspects detained during a dozen raids across the country on Thursday against an Islamist group and a further two people targeted by the investigation were held in France, state prosecutors said.Police in France arrested a dozen people suspected of helping the Islamist militant gunmen in last week's Paris killings, the city prosecutor's office said on Friday as US Secretary of State John Kerry arrived for talks.The arrests came after Belgian police killed two men who fired on them during one of about a dozen raids on Thursday against an Islamist group and German police said they had arrested two people following a raid on 11 properties linked to radical Salafists.The arrests in France centered on southern Paris suburbs including the Montrouge area where a young policewoman was killed in the attacks, the arrests were for suspected "logistical support" for the shootings, an official said.A spokesman told a news conference there was no apparent link to last week's Islamist attacks in Paris and the identities of two gunmen killed during one of the raids in Belgium, in the eastern town of Verviers, had yet to be confirmed.As well as guns and explosives, police uniforms were found in the apartment at Verviers, he said. Officials have said they feared the group was about to launch attacks on police stations. Investigators are still poring over the complex chain of events that led to three French nationals - two brothers with Algerian roots and a third of African extraction - perpetrating the worst attacks on French soil for decades.Belgian investigators said they are trying to establish if a man detained in the city of Charleroi on suspicion of arms trafficking had any links with Amedy Coulibaly, the gunman who killed four Jews at a kosher supermarket in Paris last week.His lawyer Michel Bouchat told French media the man was not an associate of Coulibaly and had merely sold him a car. The man in question already contacted police on Tuesday to say he had had contacts with Hayat Boumedienne, the partner of Coulibaly now believed to be in Syria.