The Kremlin on Thursday accused opposition leader Alexei Navalny of working with the CIA and making "groundless and unacceptable statements" after he claimed President Vladimir Putin had orchestrated his poisoning with Novichok.
"I assert that Putin is behind this act, I don't see any other explanation," the anti-corruption campaigner told Der Spiegel in his first published interview since being discharged from a German hospital.
But Putin's spokesman Dmitry Peskov countered: "We believe that such charges against the Russian president are absolutely groundless and unacceptable."
He also claimed that the CIA was "currently working" with Navalny, who studied at Yale University, but did not provide any evidence.
"It's not the patient who is working with Western security services, it's Western security services who are working with him," the Kremlin spokesman said.
"I can even say directly that CIA instructors are currently working with him."
Navalny immediately said he would sue Peskov, accusing him of "talking nonsense."
The 44-year-old also challenged the Kremlin to release "evidence and facts" proving his cooperation with the CIA.
"If the authorities on behalf of which Peskov is speaking have the proof of the nonsense he's saying, then it's a matter of national security, and I demand this proof be published," Navalny said.
Navalny was evacuated to Berlin for treatment after he collapsed in August on a flight from Tomsk to Moscow following a campaign trip to support opposition candidates in local elections.
The Putin critic was discharged just over a week ago after Germany found that he was poisoned with the Soviet-designed nerve agent Novichok. The Kremlin has denied any involvement.
In the interview with Der Spiegel, Navalny again vowed to return to Russia as soon as he has fully recovered, saying he would not give Putin "the gift" of his absence from the country.
Earlier Thursday, the speaker of Russia's lower house of parliament, Vyacheslav Volodin, also accused the "shameless" opposition leader of working for Western security services and claimed Navalny owed his life to Putin.
"Navalny is a shameless and mean man," Volodin said in a statement.
"Everyone — from pilots to doctors to the president — were sincerely saving him."
"Only a dishonorable man can make such statements," he said in an apparent reference to Navalny's claim that Putin was behind the poisoning.
"It is absolutely obvious that Navalny is working with the security services and authorities of Western countries."
Navalny's supporters organized his medical evacuation after Russian doctors treating him said they found no evidence of poisoning and suggested he had a problem with his metabolism.