Stoltenberg added his voice to a growing international chorus urging Russia to probe the incident, in which Navalny's supporters say he was poisoned by a cup of tea at a Siberian airport.

Navalny, one of President Vladimir Putin's fiercest critics, is being treated in Berlin, where doctors said clinical tests showed he had been poisoned -- a finding rejected by the Kremlin.

"We have no reason to doubt the findings of the doctors in the hospital," Stoltenberg said as he arrived for talks with EU defence ministers in Berlin.

"What we need now is a transparent investigation to find out what happened and to make sure those responsible are held accountable."

Russia has not opened a criminal investigation into the poisoning and Kremlin spokesman Dmitry Peskov said there was "no pretext" for this until the toxin is identified.

Navalny is the latest in a long line of Kremlin opponents to fall seriously ill or die from poisoning, either suspected or proven.