The Estonian National Defence Council, an advisory body to the Estonian President, convened on 16 September to discuss the matter.

"The abduction of Estonian citizen Eston Kohver, an official of the Estonian Internal Security Service, on Estonian territory and his removal to a Moscow prison violates international law. Estonia and its allies will do everything they can to effect the return of Mr Kohver," President Toomas Hendrik Ilves said at the meeting.

The members of the National Security Council were given an additional overview of events on 5 September, when Eston Kohver was in the line of duty and collecting information on cross-border corruption within the framework of an information collection plan in the vicinity of the Estonian-Russian border.

Estonian government has hired two Russian advocates, Mark Feigin and Nikolai Polozov, to represent the abducted Estonian.

"It is the worst possible scenario for Estonia," Feigin said in an interview with Eesti Päevaleht.

The first court hearing concerning the arrest of Eston Kohver is due to take place in Moscow on 18 September.

According to the Estonian Interior Minister Hanno Pevkur, as far as the Estonian authorities are concerned, Kohver is not subjected to physical torture in the hands of FSB at the prison – but psychological pressure has not been ruled out.

"Anyone who is familiar with the Russian judiciary and prison system could imagine that it’s not exactly the most pleasant place to get stuck in. The pressure that will be applied to Kohver at Lefortovo, is not necessarily physical. It is certainly very concerning situation," Pevkur said to Eesti Päevaleht.

Lefortovo is a notorious prison that during the Stalin’s Great Purge was used by NKVD for interrogations with torture. The detainment of political prisoners continued under KGB and in 2005 it was handed to secret police FSB – the Russian successor of KGB.

Alexander Litvinenko, a former officer of the FSB, who was assassinated in London in 2006, recalled his confinement at the prison as "spiritually crushing."

"Lefortovo crushes you spiritually. There is some negative energy coming from those walls. They say that birds avoid flying over it. Perhaps it’s the legacy of the old days when Lefortovo was a place of mass executions and torture," Litvinenko said to Alan Cowell, the author of The Terminal Spy.

The Estonian Foreign Ministry has repeatedly summoned Russian Ambassador to Estonia for an explanation. In one of the meetings, the Russian ambassador was given a letter from Foreign Minister Urmas Paet to his counterpart in Russia - Foreign Minister Sergey Lavrov.

"Taking an Estonian citizen to Russia against his will and detaining him in a Moscow prison is a clear violation of international law," Paet said. "Eston Kohver must immediately be released so that he can return to Estonia," he added.

Paet stated that it is extremely regrettable that the bilateral contacts between Estonian and Russian authorities have not managed to lead to a final settlement for the release of Kohver thus far.

However, Lavrov’s response thus far has confirmed Russia’s official line – that the Estonian security officer was arrested on Russian territory.

"We have shown proof that Eston Kohver was abducted from the Estonian territory," Hanno Pevkur said, commenting Russia's stance. "If Russia maintains that Kohver was arrested in Russia, then it’s a war of words," he added.

According to Paet, Estonian authorities are doing all they can to achieve the release of Kohver.

"Almost 30 countries have expressed their support for us and demand the release. We will work together with our allies in the European Union and NATO to set Eston Kohver free," Paet said.