The Kremlin on Thursday said that "such accusations against the Russian president are absolutely groundless and unacceptable."

"There was a meeting, but one shouldn't call it secret,'' Navalny said in a tweet.

A German military laboratory previously confirmed the substance in his samples.

Navalny, who is being treated in a Berlin hospital, was airlifted to Germany after falling ill on a Russian domestic flight last month.

German authorities said Wednesday that tests showed “proof without doubt” that Alexei Navalny had been poisoned with a chemical nerve agent from the Novichok group.

Officials say investigators In Siberia have been working on "establishing all the circumstances of the incident."

Alexei Navalny's allies say the Kremlin is behind the illness of its most prominent critic, accusations that officials denounced as ``empty noise.''

"The circumstances of what led to Alexei Navalny's critical condition haven't yet been clarified,'' said the German government's co-ordinator for Eastern European affairs.

Navalny was in a coma in a Siberian hospital on Thursday after drinking a cup of tea that his spokeswoman said she believed was laced with poison.

The West has accused the GRU of running a global hacking campaign, targeting institutions from sports anti-doping bodies to a nuclear power company and the chemical weapons watchdog.