Germany's security situation is "very serious" amid the Islamic State group's threat to attack the country, the head of the domestic intelligence agency (BfV) told a Sunday newspaper, according to Reuters. Hans-Georg Maassen reportedly said that despite concerns, authorities are still unaware of the militant group's plot to strike.

The extremist group, also known as ISIS, reportedly released a video Tuesday, suggesting it was planning further attacks in the West after the March 22 bombings in Brussels that killed 32 people, and last year's Nov. 13 Paris attacks that killed 130 people. In the video, the group mentioned possible targets, which included London, Berlin and Rome.

Maassen reportedly told German newspaper Welt am Sonntag that ISIS wanted to carry out attacks against Germany and German interests, but "at the moment we don't have any knowledge of any concrete terrorist attack plans in Germany." He also added the threat was a way of ISIS propaganda through which the Sunni-militant group, which controls large parts of Syria and Iraq, aimed to encourage supporters to stage attacks in Germany.

Maassen said that authorities have come across several cases linking Germans returning from Syria to carry out attacks in the country. He also warned that the danger posed by the jihadists from Germany remained "virulent," according to Reuters. He revealed that about 1,100 supporters were seen as potential threats in the country.

On Thursday, ISIS posted photos of Western targets online and called for a Brussels-style attack in Germany, Reuters reported, citing information gathered by the SITE intelligence group. The group also called for Muslims in Germany to carry out attacks on Chancellor Angela Merkel’s office in Berlin and the Cologne-Bonn airport.

Experts have estimated that approximately 20,000 foreign fighters are currently in Syria and Iraq. About 4,000 of those fighters came from Western Europe, including 500 to 600 from Germany. Britain, France and Belgium have also seen high recruitment numbers, said a report from the International Center for the Study of Radicalization and Politics at King’s College in London released at the end of January.

Maassen reportedly said Sunday that his agency was aware of about 300 attempts made by Islamic fighters to recruit refugees, who were fleeing the war-torn Syria and trying to enter Europe.

"I'm particularly concerned about the many unaccompanied minors — this group is being deliberately targeted," he said, according to Reuters, adding that he saw a "huge radicalization potential" in these attempts to recruit people.