US President Joe Biden's national security advisor said Washington had warned the Kremlin that it would be "held accountable by the international community" if Navalny died.
Navalny, one of Russian President Vladimir Putin's most prominent critics, was arrested on January 17 for alleged parole violations after returning from Germany, where he had been recovering from being poisoned with a military-grade nerve agent.
He was later sentenced by a Russian court to a three-and-a-half-year sentence, though his lawyer said he would serve only two years and eight months in jail because of time he has already spent under house arrest.
In late March, Navalny launched a hunger strike to protest the authorities’ failure to provide proper treatment for his back and leg pains.
Earlier this month, he was moved to a medical facility to be treated for a possible respiratory illness.
On Saturday, Navalny's doctors warned that he could die "any minute" , pointing to the opposition politician's high potassium levels and saying Navalny should be moved to intensive care.
France, Germany and the European Union on Sunday joined a growing international chorus of protest at Navalny's plight, and EU foreign ministers will discuss the situation on Monday.
