Maria Butina, a 30-year-old Russian national and gun rights activist, was arrested by the FBI last July and charged with conspiring to act as a "foreign agent".

Judge Tanya Chutkan read out Butina's sentence after a hearing at the US District Court for the District of Columbia on Friday.

Chutkan said Butina's actions threatened US national security, with the Russian national to be deported to Russia immediately after serving out her 18-month sentence.

Butina reportedly told the court that she was "deeply sorry" for failing to register as a foreign agent, and that she regretted that her actions had harmed Russian-US bilateral relations. Butina maintained that she acted alone. Prior to sentencing, the prosecutor urged the court to show leniency in sentencing due to Butina's guilty plea and her cooperation with authorities during the proceedings.

Commenting on the sentence, the Russian Embassy in Washington, DC said in a statement that Butina is a political prisoner in the United States and demands that the US authorities immediately release her from prison.

"Butina is a political prisoner, a victim of provocations of the Secret Service and arbitrary use of repressive laws in the United States," the Russian Embassy said. "We insist on her innocence [and] demand her immediate release."

Butina, who has been in US custody since last July, pleaded guilty to one count of conspiring to act as an unregistered foreign agent as part of a plea deal in December. Last month, prosecutors agreed with the defence to deport Butina immediately after sentencing, before changing their strategy just days before Friday's hearing and recommending prison time, citing an FBI declaration about the "significant damage to the United States" that Butina's alleged work for the Russian government may have caused.

Russian President Vladimir Putin has called Butina's arrest and indictment unjustified, because she was not charged with any actual mission on behalf of the Russian government. The Russian foreign ministry has accused US authorities of pressuring Butina into giving a false confession to the charges against her, citing intolerable detention conditions, including long periods in solitary confinement, and threats of a long prison term.