Russian opposition leader Alexey Navalny has died at the Arctic prison colony where he was serving a 19-year sentence.

News of his death was reportedly issued by Russia’s federal penitentiary service in a statement.

According to Al Jazeera, the initial statement said, “Navalny felt unwell after a walk, almost immediately losing consciousness. The medical staff of the institution arrived immediately, and an ambulance team was called. All necessary resuscitation measures were carried out, which did not yield positive results. The causes of death are being established.”

It also claimed an ambulance arrived to try to resuscitate him, but he died.

Mr Navalny, 47, was an outspoken critic of President Valdimir Putin, including his decision to invade Ukraine in February 2021.

He had 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.

He was convicted of fraud and other charges that he says were trumped up to silence him.

He was recently moved to the IK-3 penal colony in the village of Kharp, in the Yamalo-Nenets region.

Al Jazeera says IK-3 is known as the “Polar Wolf” colony due to its location about 60km above the Arctic Circle – it is considered one of the harshest prisons in Russia, housing inmates convicted of grave crimes.