The Kremlin on Thursday accused opposition politician Alexei Navalny of working with the CIA after Navalny said he believed President Vladimir Putin was behind the suspected poisoning that put him in a coma in Germany.

The accusation from Kremlin spokesman Dmitry Peskov appears to be the first time the Russian authorities have directly accused Putin’s highest-profile critic of working with a foreign intelligence agency.

It raises the stakes should Navalny fulfil a pledge to return to Russia, and may add to pressure on Western leaders, especially German Chancellor Angela Merkel, to take action over his case.

Navalny responded by saying he would sue Peskov over the accusation, challenging him to present evidence to back it up. His aide Lyubov Sobol called the allegation “complete gibberish”.

Navalny, 44, emerged in recent weeks from a coma after suddenly falling ill in during a flight in Siberia and being air-lifted to Berlin for treatment. German doctors say he was poisoned with novichok, a rare Russian nerve agent.

“I believe that Putin is behind the crime and I don’t have any other versions of what happened,” Navalny told Germany’s Der Spiegel magazine in an interview published on Thursday.

Peskov said Navalny’s allegations against Putin were unacceptable, groundless and insulting.

“It’s not the patient working with Western intelligence, it’s Western intelligence working with him. That would be more accurate. There is such information. I can even say definitely – specialists of the U.S. Central Intelligence Agency are working with him currently,” the Kremlin spokesman said.

In response, Navalny wrote on his website: “Firstly, I am suing Peskov.”

“And secondly, I demand the publication of proof and facts, demonstrating my ‘work with CIA specialists’. Show it on television at prime time. You have my permission.”

You know you are dying

Germany and other Western countries have demanded an explanation from the Kremlin for Navalny’s illness. Russia says it has seen no firm evidence he was poisoned and denies involvement in any attack on him.

In a separate development, Russian news agency RIA said Moscow had invited experts from the Organisation for the Prohibition of Chemical Weapons (OPCW) to come to Russia to cooperate on the Navalny case.

Germany requested the OPCW’s assistance last month. Russia has criticised both Berlin and the agency for what it says is their refusal to share information.

Navalny has posted on social media about his recovery, but the interview with Der Spiegel was the first he has given to a major news organisation since regaining consciousness.

He told Der Spiegel he would return to Russia.

“My task is now to remain fearless. And I have no fear! If my hands are shaking, it’s because of the poison, not out of fear. I will not give Putin the gift of not returning,” he said.

Describing the moment that the poison began to take effect, Navalny said: “You don’t feel any pain but you know you’re dying.” He said he was making a steady recovery and was now relearning how to balance on one leg.

Navalny has tormented the Kremlin for years with online videos in which he details the lavish lifestyles and undeclared wealth of senior officials he castigates as “swindlers and thieves”.

In a country where opposition figures have little access to traditional politics, his anti-corruption campaign has become one of the few successful outlets for popular discontent.

He and his wife are now living in a rented flat in Berlin, but he said he would only resume posting videos once he was back in Russia. “I do not want to be the opposition leader in exile.”

The politician, who was visited at his hospital bedside by Merkel, said he felt a personal bond to Germany and was impressed by Merkel’s knowledge of Russia.

“My impression is that Merkel needs no advice from me,” he said. “But any Russia strategy must account for the depth of madness that Putin has now reached.”