Russia's prison service said Thursday that top Kremlin critic Alexei Navalny faces immediate arrest once he returns from Germany.

Navalny, who has been convalescing in Germany from an August poisoning with a nerve agent that he has blamed on the Kremlin, said he will fly back home Sunday. He accused Russian President Vladimir Putin of now trying to deter him from coming home with the threat of arrest. The Kremlin has repeatedly denied a role in the opposition leader’s poisoning.

At the end of December, the Federal Penitentiary Service, or FSIN, warned Navalny that he faced time in prison if he fails to immediately report to its office in line with the terms of a suspended sentence and probation he received for a 2014 conviction on charges of embezzlement and money laundering that he rejected as politically motivated. The European Court for Human Rights had ruled that his conviction was unlawful.

The FSIN said in a statement Thursday that it issued an arrest warrant for Navalny in late December after his failure to report to its office. The prison service, which has asked a Moscow court to turn Navalny's 3 1/2-year suspended sentence into a real one, noted that it's “obliged to take all the necessary action to detain Navalny pending the court's ruling."

In a parallel move just before New Year's, Russia’s main investigative agency also opened a new criminal case against Navalny on charges of large-scale fraud related to his alleged mishandling of $5 million in private donations to his Anti-Corruption Foundation and other organizations. Navalny has also dismissed those accusations as crudely fabricated.

Navalny, the most visible Putin critic who had received numerous brief jail terms over the past years, fell into a coma while aboard a domestic flight from Siberia to Moscow on Aug. 20. He was transferred from a hospital in Siberia to a Berlin hospital two days later.

Labs in Germany, France and Sweden, and tests by the Organization for the Prohibition of Chemical Weapons, established that he was exposed to a Soviet-era Novichok nerve agent.

Russian authorities insisted that the doctors who treated Navalny in Siberia before he was airlifted to Germany found no traces of poison and have challenged German officials to provide proof of his poisoning. They refused to open a full-fledged criminal inquiry, citing the lack of evidence that Navalny was poisoned.

Last month, Navalny released the recording of a phone call he said he made to a man he described as an alleged member of a group of officers of the Federal Security Service, or FSB, who purportedly poisoned him in August and then tried to cover it up.

The FSB dismissed the recording as fake.