A Russian court on Wednesday began hearing a treason case against a former military engineer who is accused of giving away state secrets to a Swedish company in a letter seeking employment.

The Moscow City Court held a preliminary sitting into whether Gennady Kravtsov committed treason, his lawyer Ivan Pavlov said. The entire case is classified and will be closed to the media.

Kravtsov quit his job as a military engineer in 2005 and then applied for work abroad, Pavlov said.

"He sent a letter in 2010 to one Swedish organisation asking if they would be interested in inviting him to work," he told AFP. The defence argues that the information in the letter did not contain state secrets.

Kravtsov was born in 1968 and has three children, Pavlov added.

Some Russian media have said that Kravtsov was working for the Main Intelligence Directorate (GRU), Russia's military intelligence agency.

He has been held in the notorious Lefortovo jail, used for sensitive cases, since May 2014, and the court on Wednesday has extended his detention until January 10.

Russia has charged an increasing number of its citizens with high treason in recent months amid heightened tensions with the West over the Ukraine conflict.

Separately, a spokeswoman for another court in Moscow told AFP that three men facing treason charges -- Yevgeny Chistov, Viktor Shure and Maxim Lyudomirsky -- had their detention extended in hearings Wednesday.

No details could be given out about the cases as they had been classified as secret, the spokeswoman said.

Among other high-profile treason cases was that of a provincial housewife and mother of seven who phoned the Ukrainian embassy worried that soldiers stationed in her town may be sent there. The charges were later dropped after a public outcry.