MOSCOW — Russian opposition leader Alexei Navalny was poisoned with a nerve agent similar to Novichok, the German government said Wednesday, citing "unequivocal" evidence of the presence of the toxin.

Novichok, a class of chemical weapons developed by the former Soviet Union and Russia, was used to poison former Russian double agent Sergei Skripal and his daughter in Salisbury, England, in 2018. Western intelligence blamed Russian agents for that poisoning.

“Now it is evident: Navalny is a victim of a crime. He was supposed to be silenced,” German Chancellor Angela Merkel said at a news conference Wednesday, adding that she condemned the incident in the “strongest possible terms.”

“The world is expecting answers,” Merkel said. “The crime against Alexei Navalny is a crime against the basic values and basic rights that we stand for.”

Navalny, the fiercest critic of Russian President Vladimir Putin, fell ill on a flight to Moscow from Tomsk in Siberia on Aug. 20, and aides said they were sure that he had been poisoned. He was rushed to a hospital in the Siberian city of Omsk after the plane was diverted.

Navalny was flown to Charité hospital in Berlin two days later, with his family and aides accusing Russian doctors of blocking his transfer for 48 hours. He remains at Charité in a medically induced coma.

German doctors announced Aug. 24 that he showed symptoms of poisoning with a cholinesterase inhibitor, a class of chemical agents that includes Novichok and other nerve agents. But Merkel said Wednesday that a German military hospital had found “unequivocal” evidence of a chemical nerve agent of the Novichok group.

The revelation is likely to increase tensions between Russia and the West. Merkel’s government has long been under pressure from the Trump administration and some members of her own party to take a harder line against Moscow. Germany said it is seeking a Europe-wide response to the poisoning.

British Foreign Secretary Dominic Raab said he was “deeply concerned” that Navalny was “poisoned by Novichok, a nerve agent used with lethal effect” in Britain.

“The Russian government has a clear case to answer,” Raab tweeted.

Dmitry Peskov, a spokesman for Putin, said Russia would cooperate with Germany on the matter but that German authorities had not responded to earlier communications from Russia’s prosecutor general and doctors about Navalny. Russia maintains that no toxic agents were found in Navalny’s system before his departure to Berlin.

After Navalny fell ill, Russian doctors ruled out poisoning, suggesting that he had a metabolic disorder.

Russian lawmaker Leonid Slutsky, the head of the foreign affairs committee in the State Duma, on Wednesday called on German authorities to substantiate their claim of poisoning.

Navalny remains in “serious condition” in an intensive care unit, Charité hospital said Wednesday.

“Recovery is likely to be lengthy,” Charité said, and that it was still too early to gauge the long-term effects of the poison.

The development put Merkel in a thorny position as she pushes ahead with Nord Stream 2, a controversial pipeline project that will carry natural gas from Russia to Germany. She has resisted calls to cancel the project, despite sanctions related to the effort from the Trump administration.

Germany also is still awaiting a response from Moscow regarding a killing in Berlin last year that Germany said was linked to intelligence agencies from Russia or the Russian republic of Chechnya. Germany expelled two Russian diplomats after the killing.

Navalny is Russia’s most prominent opposition figure and is known for YouTube exposés of corruption and graft among Russian politicians, bureaucrats and oligarchs.

He was barred from running in the 2018 presidential election and has been frequently jailed for organizing unsanctioned protests.

On the Siberia trip during which he was poisoned, Navalny was filming another video on alleged corruption by members of Putin’s United Russia party, as well as supporting local candidates in forthcoming regional elections.

In March, authorities froze his bank account and those of his family members, including his parents, his daughter and even his 11-year-old son.

He is the latest in a succession of Kremlin critics suspected or confirmed to have been poisoned. These include the crusading journalist and Kremlin critic Anna Politkovskaya, who fell ill on a flight in 2004 after drinking tea and believed she was poisoned. She survived but was shot dead outside her apartment in 2006.

Alexander Litvinenko, a former Russian intelligence agent living in exile in London, was fatally poisoned with polonium-210 in 2006 while drinking tea in a hotel in the British capital. A British inquiry later implicated Putin in the killing.

In 2018, Skripal and his daughter, Yulia, nearly died when suspected Russian agents poisoned them with Novichok.

Pyotr Verzilov of the political activist group Pussy Riot is another possible poisoning victim. Verzilov fell ill after a suspected poisoning and, like Navalny, was evacuated to Berlin’s Charité hospital for treatment. Doctors there said that it was “highly plausible” that he had been poisoned but that they could not identify a substance.

The husband of the prominent Navalny associate Lyubov Sobol was stabbed with a syringe outside the couple’s home in 2016 and injected with a substance that sent him into convulsions. He survived.

Other critics of the Kremlin, including prominent opposition leader Boris Nemtsov, have been killed. Nemtsov was shot on a bridge near the Kremlin in 2015.

Morris reported from Berlin.