Following the attacks in Paris and Brussels, European security authorities are searching for at least eight further suspects, Germany's Die Welt newspaper reported in its Sunday edition.
The Islamists, mostly French and Belgian, are believed to be on the run in Syria or within Europe, the newspaper reported, citing security sources.
The terrorist attacks in Paris on November 13 left 130 people dead, while the most recent attacks on March 22 in Brussels left 31 people dead.
The suspects are all believed to have been contact people for the Paris mastermind, Abdelhamid Abaaoud, and his accomplice, Salah Abdeslam, who was recently arrested in Belgium.
In Germany, the Federal Office of Criminal Investigation (BKA) is involved in the anti-terrorist operation.
Results of investigations in France and Belgium showed that the presumed head of Islamic State's European terrorist cell was the Algerian Mohamed Belkaid, who was killed in a Belgian police raid on March 15.
Together with Najim Laachroui, the suicide bomber at Brussels airport, Belkaid is thought to have directed by telephone the terrorists who committed the atrocities in Paris.
Meanwhile, a man detained in the Brussels neighbourhood of Schaerbeek on Friday afternoon has been placed under arrest and charged with participation in the activities of a terorrist group, the Belga news agency quoted prosecutors as saying on Sunday.
The man, identified only as Abderamane A, was taken into custody in connection with the investigation that led to the arrest in France on Thursday of 34-year-old Reda Kriket, a man said to have been in the advanced stages of planning a new terrorist attack.
Meanwhile, a Swedish woman reported missing after the Brussels attacks has been confirmed among the dead, the Swedish Foreign Ministry said Sunday.
The deceased woman, who was in her 30s, was at a central metro station targeted in the explosions, spokeswoman Cecilia Gillberg told dpa.
Another Swedish woman, who was in her 60s, was confirmed among the dead late Saturday.
All Swedish nationals have now been accounted for, Gillberg added.