The German government voiced concern on Tuesday that the Islamic State group could step up attacks in Europe as it loses territory in Iraq and Syria, and said its domestic intelligence agency is training to respond to a large-scale assault.

Interior Minister Thomas De Maizière welcomed gains made by a U.S.-led coalition against the Islamic State group (also known as ISIS) in Iraq and Syria, but said they were not diminishing the risk of attacks in Europe.

"On the contrary, we fear that Islamic State will externalize, transfer its activities to Europe, especially because of military losses in the region," the minister, a member of Chancellor Angela Merkel's conservative Christian Democratic party, told reporters.

Germany has been on high alert for possible large-scale militant incidents — potentially including military-style weapons — since the ISIS attacks in Paris last November and Brussels in March, Hans-Georg Maassen, the head of the BfV domestic intelligence agency, told the same news conference.

He said the agency had carried out several exercises to prepare for such events, and several attacks had already been thwarted. Three Syrian men were earlier this month suspected of planning large-scale attacks in Düsseldorf.

Maassen said the agency was also vigilant for potential lone-wolf attacks, self-radicalized individuals, and possible militants smuggled in under the cover of over 1 million mostly Muslim refugees that have entered Germany over the past year.

He said authorities had identified clear evidence against 17 individuals who had entered Germany disguised as refugees, and most were either dead or had been arrested. "We must keep a particularly close eye on this group of people," he said.

Authorities were checking tips about a total of 400 potential Islamists among the refugees, but most of those had turned out to be false claims made by other refugees, he said.

De Maizière said the rate of Islamists leaving Germany to join ISIS in Iraq and Syria had slowed, but remained troubling. A total of 820 such people were now believed to have departed Germany for the region, up from 780 at the end of December, with about one-third estimated to have returned.

About 60 of those who had tried to reach the region were under the age of 18, and 20 of those who actually succeeded were girls, Maasen said.

At the same time, Germany has seen sharp increases in the number of ultra-conservative Islamists known as Salafists in recent years, with the total number of sympathizers now seen at 8,900, up from 7,000 at the end of 2014, German officials said.

De Maiziere said it was important to reintegrate the so-called "foreign fighters" who returned to Germany, some of whom were highly radicalized, while others were disillusioned.