'Serious questions' for Russia: World leaders react with horror to reports of Putin critic Navalny's death

• NATO Secretary General Jens Stoltenberg said he was "deeply saddened and concerned" about the reports of Navalny's demise.
• The Kremin critic appeared in court via videolink earlier in the week, seemingly in good health and spirits despite the notoriously brutal conditions of the penal colony in which the 47-year-old was being held.
• "Whatever story they tell, let us be clear, Russia is responsible, and we will have more to say on this later," U.S. Vice President Kamala Harris told the Munich Security Conference on Friday.

World leaders on Friday reacted with dismay and suspicion to reports of the death of prominent Russian opposition leader Alexei Navalny in a Siberian penal colony.

Russian media reported, citing the prison service of the Yamalo-Nenets region where Navalny had been serving a lengthy sentence on what were widely considered to be politically motivated charges, that he had fallen ill and collapsed after a walk.

Navalny's team said they had yet to receive confirmation of the death and questioned the veracity of the announcement.

U.S. Vice President Kamala Harris told the Munich Security Conference on Friday that Washington was working to confirm the "terrible" news.

"Whatever story they tell, let us be clear, Russia is responsible, and we will have more to say on this later," she added.

Russia's Foreign Ministry said the response to Navalny's reported death from Western leaders is "self-exposing" at a time when no forensic medical examination is yet available.

NATO Secretary General Jens Stoltenberg said he was "deeply saddened and concerned" about the reports of Navalny's demise.

"All the facts have to be established and Russia has serious questions to answer," he told reporters.

"Alexei Navalny has been a