Estonia has had frosty relations with Russia in recent years
A former Estonian defence ministry official who sold Nato secrets to Russia has been sent to jail for 12 and a half years after a closed trial.
Herman Simm, a former head of security, pleaded guilty to treason on Wednesday.
The court where he was tried did not reveal which country he spied for, but investigators said Mr Simm passed nearly 3,000 documents to Russia.
They said he received 1.3m kroons (£73,000; $106,000) for the data. The Kremlin denied any involvement.
Nato made no comment, but the case, which is Estonia's biggest spy scandal since the Cold War, is seen as an embarrassment for the former Soviet state. Estonia joined Nato in 2004, angering Russia.
Investigators said Mr Simm, who used to be Estonia's police chief, had been working for the Russian intelligence service (known as SVR) since 1995, when he joined the Estonian defence ministry.
He became head of security at the ministry in 2000, handling classified information regarding Nato's communications and surveillance systems.
The investigators added Mr Simm met his SVR handlers, named as Valery Zemtsov and Sergei Yakovlev, three or four times a year in different European countries.
Mr Simm, 61, was arrested last September.
An international arrest warrant has been issued for Mr Yakovlev, who also uses a fake Portuguese identity.
Jaanus Rahumagi, the head of the Estonian parliament's security affairs committee, said: "I believe Simm was happy to be involved with such a big game. He liked to be undercover and secretive."
Mr Simm, who co-operated with the investigation, has been ordered to pay 20.2m kroons in damages to the Estonian defence ministry.