The head of Germany's BfV domestic intelligence agency has said "Islamic State" members have slipped in with refugees, but that there was no intelligence indicating a "concrete, imminent" attack.

"We have repeatedly seen that terrorists ... have slipped in camouflaged or disguised as refugees. This is a fact that the security agencies are facing," Hans-Georg Maassen (pictured) told public broadcaster ZDF television on Friday.

"We are trying to recognize and identify whether there are still more IS fighters or terrorists from IS that have slipped in," he said.

Europe has been on edge since the November Paris attacks, in which some of the perpetrators allegedly went to Syria. Last year, German authorities cancelled a soccer match in Hannover and on New Year's Eve Munich's train station was closed due to security threats.

Maassen said that despite the absence of intelligence on an imminent attack authorities were on high alert as jihadist cells could be planning attacks.

Watch video 01:26 German police arrests terror suspects plotting attack in Berlin

"We are in a serious situation and there is a high risk that there could be an attack. But the security agencies, the intelligence services and the police authorities are very alert and our goal is to minimize the risk as best we can," he said.

The Berliner Zeitung newspaper on Friday cited Maassen as saying the BfV had received more than 100 tip-offs that terrorists infiltrated among refugees currently in Germany. However, he cautioned that many of the tip-offs were cases of defamation.

Attacks foiled in Berlin

The comments come a day after police broke up a suspected jihadist cell planning an attack in Berlin. Two Algerians alleged to have ties to the "Islamic State" were arrested in three separate raids. Among those arrested was the ringleader who was in a refugee registration facility with his family in Attendorn in the state of North Rhine-Westphalia.

Police said the man entered Germany via the Balkan route after training in Syria. On Friday, he was expected to go before a court with his wife, who is not suspected of involvement in the plot, to be deported to Algeria on an arrest warrant for membership in IS.

One of two people question but not arrested in the raid had ties to an Islamist network in Belgium, police said. The man allegedly travelled to the Molenbeek neighborhood of Brussels, where a number of the perpetrators of the Paris terror attacks had lived or stayed.

cw/ng (dpa, Reuters)