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TALLINN, ESTONIA -- Relatives and allies of an opposition politician imprisoned in Russia say they haven't heard from him in a month and are worried about his well-being.

Andrei Pivovarov, who was sentenced to four years in prison last year, was transferred from a detention center in southern Russia in December. In the following weeks, he occasionally wrote letters to his loved ones from detention centers in other Russian cities, stopovers on the way to his undisclosed final destination.

Tatyana Usmanova, Pivovarov's partner, received the last such letter on Jan. 18. In it, the politician said he was in a detention center in St. Petersburg, his hometown, and was told that he soon would be sent to a penal colony in the nearby Karelia region.

"After that, silence," Usmanova told The Associated Press.

Letters and official requests sent to prisons in Karelia and around St. Petersburg, as well as to Russia's State Penitentiary Service, yielded no results, and Pivovarov's whereabouts remain unknown, she said.

"We don't know if he's alive; if he's feeling well; if he's being tortured or abused somehow," Usmanova said. "We don't know anything. And it's extremely hard."

Amnesty International said in a statement Friday that Pivovarov's situation amounted to an enforced disappearance.

The group's Russia director, Natalia Zvyagina, called the Russian prison transfer system "dire," and urged authorities to disclose Pivovarov's location and release him. She said he is "serving an unfair sentence on politically motivated charges for a 'crime' that doesn't exist in international law."

Russia's State Penitentiary Service did not immediately respond to a request for comment from the AP.

Russian prison transfers are notorious for taking a long time, sometimes weeks, during which there's no access to prisoners, and information about their whereabouts is limited. Usmanova said convicts are transported in special train cars attached to ordinary trains and pass through detention centers in various, sometimes out-of-the-way regions.

Pivovarov was pulled off a Warsaw-bound flight at St. Petersburg's airport just before takeoff in May 2021 and taken to the southern city of Krasnodar.

Several days earlier, the opposition group he headed, Open Russia, had disbanded to protect its members from prosecution after Russian authorities designated it as an "undesirable" organization. The government cited a 2015 law that made membership in such organizations a criminal offense.

In Krasnodar, Pivovarov was accused of supporting a local candidate on behalf of an "undesirable" organization. Charges against him were based on his social media posts, and he rejected them as politically motivated and triggered by his plans to run for the Russian parliament in 2021.

He was convicted and sentenced in July, when Russia's war in Ukraine and sweeping crackdown on dissent were in full swing.

In a written interview conducted when he was behind bars in December, before his transfer from Krasnodar, Pivovarov told the AP that his arrest came as a surprise -- "to put it mildly, it's an incredible feeling when a plane is being turned around on the runway because of you" -- but that his sentence did not.

"By the summer of 2022, the political field was completely purged. Those who hadn't left ended up behind bars just like me," the 41-year-old wrote.

Despite his detention, Pivovarov still managed to run, albeit unsuccessfully, for Russia's parliament in 2021. He was one of the few opposition politicians to be allowed on the ballot, and his team ran his campaign while he was behind bars.

"Opposition activism in Russia is, by and large, the task of finding a way out of situations with no way out. They prohibit everything for you and watch what you'll do," Pivovarov wrote. "They tried to shut me up, but it had the opposite effect."

His campaign, he said, became a platform for him and his allies to speak out.

It was in jail where he learned that President Vladimir Putin launched "a special military operation" in Ukraine.

Other inmates, who had TVs in their cells, initially followed the war in Ukraine "as a movie, a football match where you should root for 'our guys,"' the politician said. But that attitude changed last summer, when Russia's Wagner Group, a private military contractor, started recruiting convicts to fight in Ukraine.

"The opportunity to go there was viewed as a chance to avoid (serving) a lengthy term," Pivovarov said, adding that inmates were largely motivated by hopelessness.

In his facility, some 150 men signed up, he said.

Asked whether there was any point in opposition activism in recent years, given that the Kremlin either imprisoned or drove most activists out of Russia Pivovarov said "there most certainly was," but he acknowledged some things could have been done differently.

"Looking back, you realize that maybe we didn't get all of the priorities straight. Focusing on corruption, authoritarianism, rights violations, we overlooked the emerging militarism," he said.

"But the fact that civil society, alternative information channels and (opposition) leaders, even behind bars or in exile, are still there -- it is something."