US warns Putin there will be consequences if activist Alexei Navalny dies in prison as doctors warn he could die any minute due to kidney damage.

The US has warned Russian president, Vladmir Putin, there will be consequences if Russian opposition politician Alexei Navalny dies in jail after doctors warned his health is failing amid hunger strike.

Navalny, an anti-corruption campaigner and Putin’s most-prominent critic, was first arrested in January upon his return to Russia following a Novichok poisoning allegedly carried out by an FSB hit squad.

He was then jailed for two and a half years the following month over an old embezzlement case, and transferred to a penal colony on February 26.

Navalny, 44, has been on hunger strike since March 31 because he says Russian prison guards are refusing him proper treatment for acute pain in his back and numbness in his legs while Moscow insists he is being given adequate care.

US national security adviser, Jake Sullivan said on Sunday April 18 that Biden is weighing up a range of responses if Navalny dies, and that Russia is aware of the threat.

Sullivan said Washington is ‘looking at a variety of different costs that we would impose… if Mr Navalny dies’, while refusing to go into specifics.

The US’s position comes after Yaroslav Ashikhmin, a doctor acting on behalf of Navalny’s family, said test results carried out in prison show dangerous levels of potassium in his blood along with signs of kidney failure.

‘Our patient could die at any moment,’ Ashikhmin warned.

On March 15 Navalny uploaded his first Instagram post from inside jail, likening conditions to ‘a concentration camp’ alongside an image of him with a shaved head.

On Saturday Biden called Navalny’s treatment ‘unfair’ and ‘totally inappropriate’ when asked about it during a round of golf.