More 'lone wolf' attacks expected, says Germany

"Like several EU countries, like the whole EU, Germany is also in the target area of international terrorism ... the situation is serious," he said, reports Reuters.

His warning followed an axe and knife assault by a 17-year old Afghan asylum seeker against train passengers in Germany on Monday.

The minor is said to have carried out the non-lethal attack on behalf of the Islamic State (IS). He was shot dead on the scene by police.

A German government spokesperson was quoted by Reuters saying that a "cruel attack carried out by an individual can't discredit a group of many thousands."

The issue is sensitive due to Germany's open-door policy for asylum seekers, amid fears that the latest attack could inflame anti-refugee sentiment.

Over 1 million people fleeing war in places like Syria entered the country last year.

Two of the attackers in the Paris shootings on 13 November had entered Greece as refugees, but the EU police agency, Europol, in a report earlier this week noted there is no evidence that terrorists "systematically" use the flow of refugees to infiltrate Europe.

Instead, it said "a real and imminent danger" is that Islamic extremist recruiters may attempt to radicalise vulnerable young Syrian refugees after they settled in the EU.

So-called lone wolves, who have had no direct contact with established terrorist groups but who carry out attacks in their name also remained a major security risk, Europol said.

The Afghan teenager in Germany appeared to fit this profile, said De Maiziere.

Another 84 people were killed and over 300 injured in Nice on Bastille Day after Mohamed Lahouaiej-Bouhlel, a 31-year old man of Tunisian origin, drove a lorry through a crowded boardwalk.

Lahouaiej-Bouhlel had moved to France in 2005 and had shown no previous interest in religious ideology prior to the attack.

The lone wolf threat is being exploited both by IS and by the al-Qaeda extremist group.

The Europol report said Internet propaganda was being used to radicalise people and to incite them to carry out attacks in their home countries.

"IS continues to use their media activities to encourage aspiring terrorists to conduct lone-actor attacks," it said.

Last year, IS spokesperson Abu Muhammad al-Adnani called upon followers to launch attacks against “crusaders" in their home countries.

IS leader Abudkar al-Baghadadi made similar comments last May.

The killings are not always linked back to attacks carried out in the name of a radical Islam.

One of the most notorious lone wolves is Anders Behring Breivik, a Norwegian far-right sympathiser, who shot dead 69 people in 2011. He also carried out a bomb attack that killed another eight people.