The husband of a Russian woman who is in custody on suspicion of espionage says he has been called for questioning.

Svetlana Davydova, a mother of seven who has been in custody since January, is suspected of treason, which carries a prison sentence of up to 20 years.

Davydova's husband, Anatoly Gorlov, says the charges relate to her call to the Ukrainian Embassy to report an overhead conversation on a bus about Russian troops in Ukraine.

Gorlov said on Tuesday he had been summoned for questioning in relation to his wife's case.

Despite the mounting evidence, Moscow denies any role in the bloody conflict in Ukraine that has killed more than 5,300 people.

Davydova's ordeal has caused outrage in Russia, with activists picketing offices of the KGB's successor on Tuesday.