An Estonian official who handed NATO and other defence secrets to Russia was jailed for more than 12 years on Thursday, in a case that has caused deep concern in the Western military alliance.

Herman Simm, 61, handed over more than 2,000 pages of information to his handlers in Russia's SVR Foreign Intelligence Service, local media quoted documents linked to the investigation as saying.

Simm worked at the Defence Ministry from 1995 to 2006 and had access to top secret documents, including those related to NATO, which Estonia joined in 2004.

A district court said in a statement it had convicted Simm of state treason and passing on classified information and jailed him for 12 years and six months.

"The activities of Herman Simm were a risk not just to Estonia but to our partners," said Mati Raidma, head of parliament's defence committee, after the verdict.

"But on the positive side, it was a good sign that our security services and our processes worked well to find him and bring him to justice," he told Reuters. Simm's wife was arrested with him last September, but was later released without charge.

Simm, who was arrested last September, was the most serious case of espionage since the former Soviet state joined NATO and deeply embarrassing for a nation which views itself as a staunch ally of the West.

Estonia quit the former Soviet Union in 1991 and along with its Baltic neighbours Latvia and Lithuania, it made integration with the West via NATO and the European Union a top priority. Ties with huge neighbour Russia remain edgy.

The prosecutor's office said last year Simm might have passed information to an officer from Russia's SVR, who had a fake European Union identity.

NATO headquarters was alarmed by the case, local media said, and alliance nations have been looking at the damage caused.

Baltic news agency BNS quoted investigation papers as saying that since September 2001 Simm passed just over 2,000 documents to his contacts at the SVR relating to Estonian defence policy, the defence forces, foreign relations and information systems.

BNS quoted the papers as saying he worked with the SVR since 1995 and also passed on documents relating to foreign countries' and international organisations' communication and information systems, data security, intelligence, counter-intelligence and defence policy.