Svetlana Davydova, 36, was arrested last week by a group of men in black uniforms who burst into her apartment in the town of Vyazma, west of Moscow, her husband Anatoly Gorlov told AFP.
She was still breastfeeding their youngest child, a two and a half month old girl, when she was taken away, he said.
The woman -- who faces up to 20 years in prison -- is being held at the high-security Lefortovo jail in Moscow, her lawyer Andrei Stebenev told AFP.
He said he could not comment further because the Russian General Staff said details of the case constituted a "state secret".
A spokeswoman for Moscow's Lefortovo district court confirmed the existence of the "secret case".
The FSB security service, successor to the Soviet-era KGB, whose investigator is in charge of the case, declined to comment.
Davydova's husband told AFP that his wife, who had taken an anti-war stance over the Ukraine conflict, phoned the Ukrainian embassy last April and apparently told them the local military base in Vyazma was empty, suggesting soldiers there had been deployed across the border, some 200 kilometres to the south.
The troops would wear plainclothes and remain away at least until elections.
Davydova suspected the troops would be deployed to Ukraine which held presidential elections last May, Gorlov said.
The fighting between Moscow-backed separatists and government troops broke out in eastern Ukraine in April.
The Kremlin has denied Russian troops have been fighting alongside insurgents.
Last year Russia charged an elderly rights activist with fraud after she challenged Moscow's denials that its troops were on the ground in Ukraine.
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