Munich, Germany - Five European countries, including Britain, France, and Germany, said Saturday that Russian opposition leader Alexei Navalny was killed by a "rare toxin" from a dart frog and that the Russian state was the prime suspect.

Navalny, a staunch critic of President Vladimir Putin, died in a Russian prison on February 16, 2024, while serving a 19-year sentence.

"The UK, Sweden, France, Germany and The Netherlands are confident that Alexei Navalny was poisoned with a lethal toxin," the countries said in a joint statement released at the Munich Security Conference.

According to the European states, a toxin found in the skin of South American dart frogs known as epibatidine was found on laboratory analyses of samples from his body.

"Russia claimed that Navalny died of natural causes. But given the toxicity of epibatidine and reported symptoms, poisoning was highly likely the cause of his death," said the statement.

"Navalny died while held in prison, meaning Russia had the means, motive, and opportunity to administer this poison to him," the countries said.

Britain's foreign office said separately that "only the Russian state had the means, motive and opportunity to deploy this lethal toxin." It added: "We hold it (Russia) responsible for his death."

UK Prime Minister Keir Starmer hailed Navalny's "courage in the face of tyranny" in a social media post, slamming "Putin's murderous intent."

The Kremlin has not yet commented on the allegations. Russia's foreign ministry spokeswoman and Moscow's embassy in London dismissed the Western report.

Putin said in 2024 that Navalny had "passed away." The opposition leader died shortly before a presidential election in Russia.