The US has warned Russia there will be "consequences" if the opposition activist Alexei Navalny dies in jail.
He started a hunger strike on 31 March in protest at not being able to see his own medical team, and his doctors say recent blood test results indicate he could suffer kidney failure and go into cardiac arrest at any moment.
US national security adviser Jake Sullivan told CNN there would be "consequences if Mr Navalny dies" and Russia would be "held accountable by the international community", while President Joe Biden said his medical treatment was "totally unfair and totally inappropriate".
On Sunday, a number of countries joined the international protest against Navalny's treatment inside the Russian jail in the town of Pokrov, about 100 km (62 miles) east of Moscow.
Also joining the call for action are more than 70 well known writers, artists and academics, who have have signed a letter calling on President Putin to ensure Navalny receives adequate medical care.
The letter was published in The Economist and France's Le Monde newspaper, and included the signatures of Hollywood actors Jude Law, Ralph Fiennes and Benedict Cumberbatch, Harry Potter author JK Rowling and director Ken Burns.
