The head of Ukraine's Defence Intelligence says Russian opposition leader Alexei Navalny died from a blood clot, denying speculation that he was killed while in prison.

Kyrylo Budanov was quoted as telling reporters on Sunday that "I may disappoint you, but we know he died from a blood clot," adding that it was "more or less confirmed."

Navalny, the outspoken critic of Russian President Vladimir Putin, reportedly died in an Arctic prison colony on February 16.

Russian officials said he died of natural causes, but there is speculation in the West that the Putin administration was responsible for his death.

In a statement on Saturday, the leaders of the Group of Seven nations called on the Russian government to clarify the circumstances around Navalny's death.

Budanov also made it clear that Ukraine will step up attacks on a bridge connecting Russia with Crimea in Ukraine's south, suggesting that the Crimean bridge will come down. Moscow unilaterally annexed Crimea in 2014.