A high-ranking Estonian defence official has been charged with treason, accused of passing sensitive NATO information to the Russian government for the past several years.

Herman Simm, 61, was arrested in the Estonian capital Tallinn in September, but details of the case have only recently emerged. Investigators from NATO and the European Union are in Tallinn.

Simm and his wife Heete, using a Cold War-era radio transmitter, allegedly offered classified information including the alliance's plans on Kosovo's independence, U.S. missile defence and the recent conflict between Russia and Georgia.

Heete, a lawyer, has been detained on charges of being an accessory to treason.

NATO hasn't commented on the charges, but one German official described the potential damage to the organization as catastrophic. Jaanus Rahumagi, a member of Estonia's parliament, told Der Spiegel he fears "historic damage."

Simm, who handled classified information technology issues within NATO, was also responsible for granting security clearances, raising the possibility he could have allowed Russian spies to infiltrate the alliance.

Officials allege he and his wife delivered the classified information using a Cold War-era radio transmitter.

His actions didn't appear to raise any suspicions, including his purchases of six valuable homes and plots of land, including a farm near the Baltic coast.

Officials say Simm, who is a former Estonian chief of police, was uncovered when his Russian handler attempted to recruit another Estonian official. That person reported the offer to NATO authorities.

Simm is expected to be arraigned early next year and if found guilty, could face up to 15 years in prison.