BERLIN (Reuters) - A German citizen employed by Germany's domestic intelligence agency has been arrested on accusations that he made Islamist declarations on the internet and revealed internal agency material, the agency said on Tuesday.

A spokesman for the Bundesverfassungsschutz (BfV) declined to provide details on the man's position at the agency or say when he joined. He also declined to comment on a report in Die Welt newspaper that the 51-year-old had planned to explode a bomb at the agency's central office in Cologne.

"There is no evidence to date that there is a concrete danger to the security of the BfV or its employees."

"The man is accused of making Islamist statements on the Internet using a false name and of revealing internal agency material in Internet chatrooms," he said.

The suspected mole also offered to share sensitive data about the BfV which could have endangered the agency's work, the spokesman said, without elaborating.

Der Spiegel magazine reported on its website that the agency first became aware of the man's activities about four weeks ago.

The spokesman said the suspect had not previously attracted attention, adding: "The man behaved inconspicuously during his employment process, training and in his area of responsibility."

German authorities have ramped up their surveillance of potential militant Islamist groups and individuals after two attacks claimed by the Islamic State group in July.

The BfV estimates there are about 40,000 Islamists in Germany, including 9,200 ultra-conservative Islamists known as Salafists, Hans-Georg Maassen, who leads the agency, told Reuters in an interview earlier this month.

"We remain a target of Islamic terrorism and we have to assume that Islamic State or other terrorist organizations will carry out an attack in Germany if they can," he said at the time.

Police have arrested several suspected Islamic State sympathizers in recent weeks, including a 20-year-old Syrian refugee who had tried to cross into Denmark with potential bomb-making materials.

(Reporting by Andrea Shalal; editing by Ralph Boulton)