Maria Butina, who pleaded guilty to not registering as an agent for the Russian government while in the United States and was sentenced to 18 months in U.S. prison, has filed an appeal against her sentence.

The 30-year-old Butina filed the appeal document from the Grady County Jail in Chickasha, Oklahoma, where she had been transferred after being held in Alexandria, Virginia.

The brief document did not state the basis for her appeal.

Butina admitted covertly gathering intelligence on the National Rifle Association (NRA) and other groups under the supervision of former Russian lawmaker Aleksandr Torshin.

But speaking from a U.S. jail by phone to Russian reporters, Butina on April 30 said she "didn't expect such a severe punishment.'

'This is absolutely absurd. This is a huge disgrace for the U.S. justice system,' she added.

The judge also ordered Butina to be deported to Russia immediately after she finishes her prison sentence, which is expected in November after receiving credit for the time she has spent in U.S. custody.

Russian President Vladimir Putin has criticized the sentence, calling it 'arbitrary' and saying Moscow authorities 'don't understand why she was sentenced.'

Butina was sentenced in April after pleading guilty to a charge of conspiracy to act as an unregistered foreign agent. Butina violated U.S. law because she did not report her activities to the U.S. government, and has been jailed since her arrest in July 2018.

She admitted that she should have registered with the U.S. government but insisted she wasn't a spy and that all her actions took place out in the open.

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