German prosecutors have arrested former intelligence chief August Hanning on suspicion of attempted treason, escalating one of the country's most serious alleged espionage cases in recent years.

Investigators say Hanning paid for access to hundreds of classified documents and used them to prepare an analysis for an unidentified foreign intelligence service.

Hanning, who led Germany's BND intelligence agency from 1998 to 2005, faces additional allegations including espionage, disclosing state secrets and acting as a foreign agent.