BERLIN (Reuters) - Some young Muslims are attracted to the Islamic State in Iraq and Syria because of its brutality, which makes it appear "more authentic" than al Qaeda, the head of Germany's domestic intelligence agency said in an interview on Sunday.

"There is a link between the successes IS has had so far in Iraq and the activities here in Germany and the propaganda and canvassing activities aimed at young jihadists," said Hans-Georg Maassen, head of Germany's BfV domestic intelligence agency.

"The Islamic State is, so to speak, the 'in' thing - much more attractive than the Nusra Front, the al Qaeda spin-off in Syria," the BfV chief told Deutschlandfunk public radio.

"What attracts people is the intense brutality, the radicalism and rigor. That suggests to them that it is a more authentic organization even than al Qaeda," he said. "Al Qaeda fades besides the Islamic State when it comes to brutality."

Chancellor Angela Merkel cites the success of IS in recruiting German jihadists as a reason for her country to make an "exception" to its post-war policy of not sending arms to areas of conflict. Her government will decide on Sunday what military aid to send to Kurdish forces in Iraq fighting IS.

German intelligence estimates that at least 400 Germans have joined the IS insurgency in Syria and Iraq. Maassen said there was evidence that five German citizens or residents had carried out suicide attacks for the insurgents there in recent months.

The BfV has estimated that there are some 43,000 Islamists in Germany, with the numbers of the ultra-conservative Salafi movement seeing particular growth. The agency says the Internet plays an important role in recruiting youngsters.

Opinion polls suggest the German public has no appetite for getting involved in Iraq's conflict and Merkel has made clear she would not send combat troops. The opposition has warned that weapons could end up in the wrong hands and demanded a debate in parliament, which is scheduled for Monday.

Maassen, asked about the impact on the domestic security situation of the conflict between Ukrainian government forces and pro-Russian separatists, said the BfV did not detect an increase in the number of Russian agents in Germany, "which has always been at a relatively high level"."But we do see the Russian services realigning their work due to Ukraine," he said. "They're not just interested in German foreign and energy policy but Germany policy on Ukraine: how Germany will decide in Brussels, for example, when it comes to sanctions against Russia."

(Reporting by Stephen Brown; Editing by Robin Pomeroy)