LONDON, United Kingdom (AFP) — Britain will consider “increasing sanctions” against Russia following findings from five European states that opposition leader Alexei Navalny was killed by dart-frog toxin in a Russian prison, UK foreign minister Yvette Cooper said Sunday.

“We continue to look at coordinated action, including increasing sanctions on the Russian regime,” Cooper told the BBC from the Munich Security Conference, where the UK, France, Germany, the Netherlands and Sweden announced findings that the Russian state was a prime suspect for poisoning Navalny two years ago.

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

The five European countries on Saturday said that a deadly toxin known as epibatidine, found in Ecuadorian dart frogs, was found on laboratory analyses of samples from his body.

Cooper told Sky News that the toxin can also be produced synthetically.

“We do know that the Russian regime has had possession of this particular chemical,” the British foreign minister said.

“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,” the European countries said in a joint statement Saturday.

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.”

Russia’s foreign ministry spokesperson and Moscow’s embassy in London dismissed the Western report.

The Kremlin has never given a full explanation for Navalny’s death, only saying he fell ill and died suddenly after taking a walk in his prison colony.

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

On Saturday, Navalny’s widow, Yulia Navalnya, said it was now “science-proven” that the Kremlin opponent had been murdered, two years after his death was announced during the same annual conference in Germany.

Navalnaya last September said that laboratory analysis of smuggled biological samples found that her husband was killed by poisoning.

Meanwhile, Ukrainian drones damaged an oil depot in Russia’s southern Krasnodar region and caused several fires, Russian officials said Sunday.