BERLIN (Reuters) - The number of Germans fighting alongside Islamic State militants in Syria and Iraq has increased sharply to 550 and around 180 have returned, the head of Germany's domestic intelligence said in a newspaper interview published on Sunday.

"We are concerned about the high number of departures. Especially in the last six weeks, it has risen further," Hans-Georg Maassen told Welt am Sonntag.

"By now we are counting 550 people, that is how many have left Germany to go to Syria and Iraq," he added. The previous number was 450 people.

Around 60 of them have been killed during fighting, with at least nine killing themselves in suicide attacks, Maassen said. "That is a sad success for the Islamist propaganda."

As with other Western European countries, Germany is struggling to stop the radicalisation of young Muslims, some of whom want to become jihadists in Syria or Iraq. Officials also worry that they might return to plot attacks on home soil.

Since Germany is part of the alliance fighting Islamic State, the country is "naturally" a target for the militants as well as al Qaeda members, Maassen said.

Intelligence services estimate that around 180 jihadists have returned to Germany after having fought in Syria and Iraq, Maassen said.

In one of the largest sweeps against alleged Islamists in Germany yet, authorities last week arrested nine men suspected of supporting militant groups in Syria and raided numerous properties in several German states.

(Reporting by Michael Nienaber; Editing by Alison Williams)