Seventeen supporters of the Islamic State extremist group entered Europe as refugees, Frankfurter Allgemeinen Zeitung reported Saturday citing German intelligence.
Most of them are either dead or in custody, the report said, adding that two of them were part of the group of terrorists who attacked Paris in November.
Hans-Georg Maassen, the head of Germany's domestic intelligence agency, was quoted as saying there was "reliable evidence" that the 17 were working on behalf of Islamic State.
He said that while Islamic State did not need to use known refugee routes it was "a show of power" on part of the terrorist group.
Last month, three Syrians were arrested in three different German states on suspicion of planning a terrorist attack in Dusseldorf on behalf of Islamic State.
The authorities said that the three had been posing as refugees and living in asylum seekers' homes.
The alleged plot foresaw two militants blowing themselves up with explosive vests in the western German city's historic centre and the remaining men killing as many people as possible with firearms.
In recent months, police have conducted a series of raids targeting Islamists sent to Germany on behalf of Islamic State.
Thus far, there has been no major terrorist attack on German soil.
Chancellor Angela Merkel's promise of sanctuary brought some 1.1 million migrants to Germany in 2015, with most from conflict zones in Syria, Iraq and Afghanistan.