Bratislava, Slovakia - US Secretary of State Marco Rubio said Sunday that Washington is "not disputing" a report by five European countries that imprisoned Russian opposition leader Alexei Navalny was fatally poisoned with a rare frog "toxin."

Rubio said the report released on Saturday by Britain, France, Germany, the Netherlands, and Sweden was "very troubling."

Britain, which has outright blamed Russia for Navalny's death, said it was considering new sanctions on Moscow as a result.

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

While Russia has never explained the circumstances, it has said Navalny died suddenly, and the foreign ministry in Moscow has dismissed the European report.

Rubio, on a brief visit to Slovakia, said Navalny's death was "very serious" and the US decision not to join the report "doesn't mean we disagree with the outcome."

The report said Navalny was poisoned with a "rare toxin," epibatidine, from an Ecuadoran dart frog. The Europeans also said that Russia was the prime suspect.

The US has held contacts with Russian officials on improving ties, but Rubio said the US administration did not join the statement because it was a European-led initiative.

"Sometimes countries go out and do their thing based on the intelligence they have gathered. We obviously were aware of the report. It's a troubling report," he added.

"Certainly we don't have any reason to question it or we're not disputing or getting into a fight with these countries over it."

The five European countries said that epibatidine was found in laboratory analyses of samples from his body.

Experts have said the toxin can also be produced synthetically, instead of extracting it directly from the Ecuadoran amphibian.