BERLIN, Dec 22 (AFP) - German authorities said on Thursday they had arrested an employee of its foreign intelligence service (BND) on suspicion of sharing state secrets with Russia this year and thereby committing treason.

Police arrested the suspect, a German citizen identified as Carsten L., on Wednesday in Berlin, the federal prosecutors office said. It said police also raided his flat and workplace as well as those of another person.

"The accused is suspected of state treason," federal prosecutors said in a statement. "In 2022, he shared information that he came by in the course of his work with a Russian intelligence agency. The content is considered a state secret."

In a separate statement, the BND said the employee had been placed in pre-trial detention and searches at two of its offices had been carried out.

German authorities have warned of likely heightened Russian spying given the Kremlin's stand-off with the West over its invasion of Ukraine. The German government expelled 40 Russian "spies" in April, according to the domestic intelligence agency.

The Russian embassy in Berlin did not immediately reply to Reuters request for comment.

The BND began its own, far-reaching internal investigation into the matter as soon as it became aware of the possibility of treason within its own ranks, BND chief Bruno Kahl said in a separate statement. When these suspicions firmed up, the BND called in federal prosecutors.

Discretion is key going forward, as any detail of the investigation made public could give the "opponent an advantage in its intent to harm Germany", Kahl said.

"With Russia, we are dealing with an actor where we must reckon with its ruthlessness and willingness to be violent."

As such, the BND would not be giving out any further details on the case until federal prosecutors had concluded their investigation, Kahl added.

The case came days after Austria said it had identified a 39-year-old Greek citizen whom it suspects of spying for Russia.

The last time a German intelligence employee was arrested for treason was in 2014 - although then it was for betraying secrets to the U.S. Central Intelligence Agency. The man, identified as Markus R., was in 2016 sentenced to eight years in prison. The episode cooled relations at the time between close allies Berlin and Washington.