UN rights chief Michelle Bachelet called on Moscow Tuesday to conduct or cooperate with an investigation into the alleged nerve agent attack on Russian opposition leader Alexei Navalny.

"The number of cases of poisoning, or other forms of targeted assassination, of current or former Russian citizens, either within Russia itself or on foreign soil, over the past two decades is profoundly disturbing," Bachelet said in a statement.

Such an investigation would have to be "thorough, transparent, independent and impartial," she said.

Navalany is a high-profile critic of Russian President Vladimir Putin. 

Moscow denies involvement

Navalny is currently coalescing in a Berlin hospital. He was removed from a medically-induced coma and is responding to speech, the Charite hospital announced Monday.

Bachelet said the medical reports from the hospitals showed that Navalny had "unequivocally" been poisoned, which Russia denies.

"It is not good enough to simply deny he was poisoned," said Bachelet.

"Navalny was clearly someone who needed state protection, even if he was a political thorn in the side of the government," she added.

Read more: Opinion: Will Vladimir Putin ride out the Navalny storm?

Not enough details yet known

Navalny was airlifted to Berlin after he fell ill on a Russian domestic flight last month. 

There was a delay in the airlift taking place, which Kremlin critics say may have been down to Russia waiting for traces of the poison to leave his system.

Bachelet said that a lack of legal processes in previous poisoning cases in Russia had led to "close to total impunity."

But she was also clear that not enough details were yet known about the poisoning.

Germany has threatened sanctions against Russia in relation to the poisoning.

ed/rt (Reuters, EPD)