The US government has described the death of Russian opposition politician Alexei Navalny as a "terrible tragedy."
Speaking to NPR radio on Friday morning, US President Joe Biden's National Security Advisor Jake Sullivan underlined the US government had not yet received confirmation of his death so was holding back on commenting for the time being.
"Given the Russian government's long and sordid history of doing harm to its opponents, it raises real and obvious questions about what happened here," Sullivan said.
