TALLINN, Estonia (AP) — Russian opposition leader Alexei Navalny said Tuesday that officials at the Arctic penal colony where he is serving a 19-year sentence have isolated him in a tiny punishment cell over a minor infraction, the latest step designed to ramp up pressure on President Vladimir Putin’s fiercest political foe.

Navalny said in a social media statement relayed from behind bars that prison officials accused him of refusing to “introduce himself in line with protocol” and ordered him to serve seven days in a punishment cell.

”The thought that Putin will be satisfied with sticking me into a barracks in the far north and will stop torturing me in the punishment confinement was not only cowardly, but naive as well,” he said in his usual sardonic manner.

Navalny, 47, is jailed on charges of extremism. He had been imprisoned in the Vladimir region of central Russia, about 230 kilometers (140 miles) east of Moscow but was transferred last month to a “special regime” penal colony — the highest security level of prisons in Russia — above the Artic Circle.

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His allies decried the transfer to a colony in the town of Kharp, in the Yamalo-Nenets region about 1,900 kilometers (1,200 miles) northeast of Moscow, as yet another attempt to force Navalny into silence.

The remote region is notorious for long and severe winters. Kharp is about 100 kilometers (60 miles) from Vorkuta, whose coal mines were part of the Soviet gulag prison-camp system.

“It is almost impossible to get to this colony; it is almost impossible to even send letters there. This is the highest possible level of isolation from the world,” Navalny’s chief strategist, Leonid Volkov, has said on X, formerly known as Twitter.

Navalny has been behind bars since January 2021, when he returned to Moscow after recuperating in Germany from nerve agent poisoning that he blamed on the Kremlin. Before his arrest, he campaigned against official corruption and organized major anti-Kremlin protests.

He has since received three prison terms, rejecting all the charges against him as politically motivated. Until last month, Navalny was serving time at Penal Colony No. 6 in the Vladimir region, and officials there regularly placed him in a punishment cell for alleged minor infractions. He spent months in isolation.

At the prison colony in Kharp, being in a punishment cell means that walking outside in a narrow concrete prison yard is only allowed at 6:30 a.m., Navalny said Tuesday.

Inmates in regular conditions are allowed to walk “after lunch, and even though it is the polar night right now, still after lunch it is warmer by several degrees,” he said, adding that the temperature has been as low as minus 32 degrees Celsius, or minus 25 degrees Fahrenheit.

“Few things are as refreshing as a walk in Yamal at 6:30 in the morning,” he wrote, using the shorthand for the name of the region.