BERLIN, Aug 24 (Reuters) - A German man held hostage for about a year in Syria by fighters of Islamic State was released in June after the government did a deal with the militant group, die Welt am Sonntag newspaper said on Sunday.

Citing German security sources, the newspaper said the Islamists, who last week released a video showing the beheading of an American journalist, were given something for the man's release but did not specify the nature of the trade-off.

The foreign ministry refused to comment on the report and the interior ministry was unavailable.

The newspaper said that a 27-year-old man from the eastern state of Brandenburg was kidnapped by IS fighters while travelling in Syria in 2013.

His family received an email earlier this year with a video showing the man alive and a ransom demand, it said.

The video also showed the execution of another hostage which the German man had witnessed, it said.

After negotiations for the release of the man, who is not identified in the report, German authorities secured his freedom in June.

Something was given in return for his release, the newspaper said without giving details. It added that the foreign ministry "denied having paid a ransom in any form".

The murder of U.S. journalist James Foley, whose beheading by an IS militant was shown in a video released last week, has heightened fears in Germany about the growing threat presented by the group, which controls large swathes of territory in Iraq and Syria.

Germany has agreed to arm Kurds in northern Iraq who are fighting IS.

Interior Minister Thomas de Maiziere told Bild am Sonntag newspaper that about a quarter of the 400 people who had travelled to Syria from Germany had returned.

"These men have learned to kill and to hate. They represent a danger to our country," de Maiziere told the paper. He said not all 100 returnees were potential terrorists or suicide bombers.

Der Spiegel magazine said German prosecutors are investigating at least 139 Germans suspected of being members or supporters of groups including IS, or of involvement in plans for violent crimes against the state.

It was unclear from the report where the individuals were or where the investigations were taking place.

(Reporting by Madeline Chambers; Editing by Lynne O'Donnell)