The two terror suspects arrested in Germany on suspicion of planning attacks in Berlin may have been sent directly by the Islamic State extremist group, sources close to the investigation told dpa on Friday.

Investigators cautioned they were still looking to back up their theory with evidence and said they are hoping to gain new insights through the analysis of computer and mobile phone data confiscated during coordinated raids across Germany on Thursday.

The raids resulted in the arrest of the alleged ringleader of an Islamist terrorist cell, a 34-year-old Algerian man, named only as Farid A in keeping with German privacy laws, his wife - thought to have acted as an accessory - and another terrorist suspect.

Two other Algerian men sought by police were encountered in the raids but not arrested.

Except for the wife, all were being investigated for "planning a violent act intended to seriously damage the state," police said.

Farid A and his wife were arrested at a gymnasium being used as refugee accommodation in Attendorn, western Germany, where he had been living with his family "inconspicuously" for several weeks.

He had used his real name and three fake ones, while his 27-year-old wife used one name aside from her own. They were posing with their two infant children as a refugee family from Aleppo, Syria, travelling to Europe along the Western Balkan route.

Photos to be released by police on Friday show the terrorist suspect receiving militant training in the Middle East. Police say the photos establish his links to a wider terrorist network.

"The photos are the reason we took the tip-off [about the planned attacks] particularly seriously," a spokesman for the Berlin police said.

The couple on Friday appeared before a judge in Hamm, a town north-east of Cologne. The judge served them a detention order, which ensures they can remain in custody in Germany until the formal process of extradition to Algeria is complete.

Another suspect was arrested in Berlin over forgery of documents. The man had left Germany in 2013 only to return the following year with forged French documents. He subsequently appeared under a number of false identities.

Police said the youngest suspect - one of the two not apprehended - has links to an Islamist network in Belgium and travelled at least once to the Brussels neighbourhood of Molenbeek, where several perpetrators in terrorist attacks in Paris had lived or stayed.

Investigators have not managed to link the suspects to terrorist threats in Munich on New Year's Eve and at a friendly football match in Hanover.

The country's domestic intelligence chief said Friday that there is no imminent threat of a terrorist attack in Germany.

"There is no evidence of a concrete, imminent terrorist crime in Germany," Hans-Georg Maassen told public broadcaster ARD on Friday.

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