Moscow (AFP) - A mother of seven accused of treason for telling the Ukrainian embassy about Russian troop movements was to be released from jail Tuesday, her husband said, though the charges against her remain.

Svetlana Davydova, who faces between 12 and 20 years in prison, is being held in the high-security Lefortovo jail in Moscow.

Her case has led tens of thousands of people to petition President Vladimir Putin to show clemency.

"The pre-trial restrictions have been changed," her husband Anatoly Gorlov told AFP, stressing however that the charge has not been dropped.

"It's too early to rejoice but the kids will see their mother."

Davydova was still breastfeeding her youngest child, a two-and-a-half-month-old girl, when she was arrested last month in the town of Vyazma, west of Moscow.

The case has shocked the country, and prompted over 50,000 Russians -- including prominent authors and TV celebrities -- to petition Putin.

"Mr President, we ask you to be merciful towards a woman and mother of a large family," the letter read.

"We are hoping that investigation and a possible trial will be as open and just as possible and will be in accordance with the norms of the law."

Among the signatories were Natalya Solzhenitsyna, widow of the Nobel literature prize-winner Alexander Solzhenitsyn, film director Andrei Zvyagintsev and actress Lia Akhedzhakova.

Putin spokesman Dmitry Peskov confirmed that the presidency had received the petition.

The US meanwhile said it was "troubled" by reports of Davydova's arrest, with the State Department urging that she "be treated in keeping with international legal norms."

Activists from the liberal party Yabloko picketed the headquarters of the FSB security service, successor to the Soviet-era KGB, whose investigator is in charge of the case.

Davydova, 36, who opposes the Ukraine conflict, phoned the Ukrainian embassy last April to allegedly report the military base located near her residential building in Vyazma had emptied, suggesting its soldiers might have been deployed across the border.

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She also purportedly informed embassy staff she had overheard a serviceman saying troops of the Russian military intelligence service, the GRU, would be sent on a mission -- presumably to Ukraine.

The Ukrainian foreign ministry said it was not clear how Russian authorities had learned of the phone call, and expressed the hope that they were not eavesdropping on its Moscow embassy.

"We would not want to think this is the case," a foreign ministry spokesman told AFP.

Observers call Davydova's case an embarrassment for authorities and decry their virtual blackout of any information regarding Russia's intervention in Ukraine.

- 'Unthinkable monstrosity' -

Critics also describe Davydova's treatment as reminiscent of the excesses of the repressive Soviet regime, and say the case signalled the government's readiness to ramp up efforts to squelch dissent.

"This is a signal of unthinkable monstrosity, this is 1938," Akhedzhakova, one of the country's most outspoken actresses, said on radio, referring to the height of Stalin-era repression.

Davydova's husband said he was grateful for the outpouring of support, adding he and other family members had been questioned by investigators earlier in the day.

"The entire family refused to testify against her."

Authorities had previously said Davydova risked losing custody of her children.

But on Tuesday Russia's ombudsman for children's rights, Pavel Astakhov, said Davydova's husband was taking good care of the children, and that they would remain together as a family.

In 2012, Russia broadened its definition of high treason, alarming rights groups that said nearly anyone could fall afoul of the amended law.

Davydova is believed to be the first high-profile victim of the modified law.

"Practically anyone can now become its victim," said Gorlov. "Now any housewife who looked out of her window can be accused of treason."