The Russian Embassy in the United States has expressed dissent over the "unlawful court verdict" in the sentencing of Russian gun-rights activist Maria Butina by the US.

"In relation to the unlawful court verdict, a note of protest has been sent to the [US] State Department. We demand the US officials to ensure the protection of rights and legal interests of our compatriot in accordance with international human rights norms," TASS quoted the embassy as saying.

Butina was sent to 18 months in prison after she pleaded guilty of allegedly trying to infiltrate conservative political circles and promoting Russian interests before and after the 2016 presidential elections.

The Embassy added that its diplomats visited Butina in prison.

"Despite the unjust and severe verdict issued on April 26, our compatriot endures the hardships of her prison sentence stoically and with dignity," the embassy said.

30-year-old Butina is so far the only Russian citizen arrested and convicted in the three-year investigation of Moscow's interference in 2016 US presidential elections.

Butina, the leader of a small Russian gun rights group used her ties to the National Rifle Association to build a network of powerful Republican contacts.

While prosecutors have admitted that Butina is not a spy in the traditional sense, they argued that her crime still could have jeopardised US' national security.

However, Butina's efforts appeared to be separate from the Kremlin's sweeping election-meddling campaign detailed in special counsel Robert Mueller's report.

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