Convicted Russian agent Maria Butina was released from a Florida prison on Friday after serving most of her 18-month sentence for conspiring to influence US conservative activists and infiltrate a powerful gun rights group, and taken into custody by immigration officials to be deported to her native country.

Key points: Russian President Vladimir Putin has previously expressed "outrage" over Butina's prison sentence and said she did not carry out any orders from Russian security services

Russian President Vladimir Putin has previously expressed "outrage" over Butina's prison sentence and said she did not carry out any orders from Russian security services Butina admitted to conspiring with a Russian official and two Americans to infiltrate the National Rifle Association in the US

Butina admitted to conspiring with a Russian official and two Americans to infiltrate the National Rifle Association in the US In 2015 she appeared at a Trump campaign event and asked him a question about whether he wanted better relations with Russia

Butina, 31, had been scheduled for release from the low-security prison in Tallahassee in early November, but a change in federal law moved up her release date based on credit for good behaviour, her attorney Robert Driscoll said.

A prison official confirmed she was released on Friday morning and taken into custody by US Immigration and Customs Enforcement (ICE) authorities.

She left Miami International Airport on a direct flight to Moscow at about 6:00 pm (local time), ICE said in a statement.

Russian President Vladimir Putin has previously expressed "outrage" over Butina's prison sentence and said she did not carry out any orders from Russian security services.

Butina, a former graduate student at American University in Washington who publicly advocated for gun rights, pleaded guilty in December to one count of conspiring to act as a foreign agent and agreed to cooperate with prosecutors.

The Siberian native admitted to conspiring with a Russian official and two Americans to infiltrate the National Rifle Association, a group closely aligned with US conservatives and Republican politicians including President Donald Trump, and create unofficial lines of communication to try to shape Washington's policy toward Moscow.

Her 18-month sentence included nine months she spent incarcerated after her July 2018 arrest.

Butina's case was separate from former Special Counsel Robert Mueller's investigation of Russian interference in the 2016 US election, which detailed numerous contacts between Trump's campaign and Russia.

Her activities occurred during the same period as the contacts investigated by Mueller.

Butina, left, next to her attorney Robert Driscoll at the US District Court. ( Dana Verkouteren via AP )

The Russian official with whom Butina conspired was later identified as Alexander Torshin, a deputy governor of Russia's central bank.

He was never charged in the case but was hit with sanctions by the US Treasury Department.

One of the two Americans referenced in her case was conservative political activist Paul Erickson, her boyfriend.

Mr Erickson was not charged for his links to Butina but was indicted on unrelated wire fraud and money laundering charges in South Dakota. The case against him is still pending.

In addition, Overstock.com chief executive officer Patrick Byrne resigned in August after confirming a report by Fox News contributor Sara Carter that he also had an intimate relationship with Butina.

Federal prosecutors have said Butina did not engage in "traditional" spycraft but worked behind the scenes to make inroads in conservative political circles and promote friendlier US-Russian relations.

She arranged dinners in Washington and New York and attended events to meet prominent politicians.

Butina in 2015 appeared at a Trump campaign event and asked him a question about whether he wanted better relations with Russia.

Trump responded by telling Butina that he would "get along very nicely with Putin."

During her sentencing, Butina reportedly begged the US judge to be allowed to return to Russia. ( Alexandria Detention Centre via AP )

Russia's Ministry of Foreign Affairs last year accused the United States of forcing Butina to make a false confession to "absolutely ridiculous charges" of being a Russian agent.

"It's not clear what she was convicted of or what crime she committed," Mr Putin said in April.

"I think it's a prime example of 'saving face.' They arrested her and put the girl in jail. But there was nothing on her, so in order not to look totally stupid they gave her, fixed her up, with an 18-month sentence to show that she was guilty of something."

Asked whether Washington now expects Russia to release a former US Marine named Paul Whelan currently being held on accusations of espionage, a State Department spokesperson said, "We continue to urge the Russian government to ensure a fair trial, including a fair and public hearing without undue delay, in accordance with its international legal obligations."

A Russian court on Thursday ordered Whelan, detained in 2018, held in custody until December 29.

Whelan, who holds American, British, Canadian and Irish passports, has denied the espionage allegations.

Reuters