Russia's Federal Penitentiary Service (FSIN) says it will seek the arrest of opposition politician Aleksei Navalny, setting the stage for a potentially dramatic new showdown between the Kremlin and Navalny, who is one of President Vladimir Putin's most outspoken foes.

The FSIN statement on January 14 comes days after Navalny was placed on Russia's federal wanted list for allegedly violating the terms of a suspended prison sentence and less than 24 hours after the 44-year-old announced plans to return this weekend from Germany, where he was airlifted for treatment after being poisoned with a Soviet-era nerve agent.

Navalny said he would return to Russia on January 17 despite having received a notice saying the FSIN had asked a court to switch his suspended prison sentence to jail time, possibly as much as 3 1/2 years.

The FSIN is "obliged to take all actions to detain" Navalny pending a court decision on turning the suspended sentence he received in 2014 into a jail term, the prison authority said in its statement.

The Kremlin critic's sentence on an embezzlement charge he says was politically motivated ended on December 30, 2020.

"I cannot say we are awaiting [his arrest] but he may actually be taken into custody at the airport," Navalny's lawyer Vadim Kobzev told reporters on January 14 before the FSIN statement.

Infographic: All The Times Aleksei Navalny Has Been In Jail

According to the website of Moscow's Simonov district court, the move to change Navalny's suspended sentence was registered on January 11.

The official reason for changing the sentence is listed as being "due to a failure to carry out imposed obligations, evading the payment of compensation, or committing another crime."

Navalny fell ill on a flight from the Russian city of Tomsk to Moscow on August 20 and was treated and placed in an induced coma in a Siberian hospital before being transferred to a clinic in Berlin.

Lab tests in three European countries, confirmed by the international Organization for the Prohibition of Chemical Weapons, have established that Navalny was poisoned with a toxin from the Russian-made Novichok group of Soviet-era nerve agents. The findings led the European Union to impose sanctions on six Russian officials and a state research institute.

Russian authorities have claimed that no trace of poison was found in Navalny's body before he was airlifted to Germany and have refused to open a criminal investigation into the incident. Navalny has blamed President Vladimir Putin directly for his poisoning.

Late last month, FSIN demanded Navalny return immediately from Germany or face jail in Russia for violating the terms of a suspended prison sentence relating to a 2014 fraud conviction and for evading criminal inspectors.

The European Court of Human Rights (ECHR) ruled in October 2017 that the Russian courts violated Navalny's right to a fair trial in the case.

His 3 1/2-year suspended sentence could be changed to a real jail term if the alleged violations are determined by Russian officials to be valid.

In a separate case, a court in Russia's Urals city of Chelyabinsk on January 14 acquitted two activists of supporting Navalny.

Oksana Yeryomina and Yury Varushin were charged with hooliganism and forcibly breaking a police cordon at a march by Navalny supporters in the city in May 2018. The action was taken to protest against Putin, who was being inaugurated on that day.

With reporting by Reuters and Meduza