The EU has said it is "deeply concerned" about reports that Kremlin critic Alexei Navalny's health was failing in a Russian penal colony and called for his "immediate and unconditional release".
The matter is on the agenda of an EU foreign ministers' video conference to be held tomorrow, EU foreign policy chief Josep Borrell said in a statement.
French Foreign Minister Jean-Yves Le Drian earlier said his country is "extremely concerned" about the health of Mr Navalny, who is on a hunger strike.
"Navalny's situation is extremely concerning," Mr Le Drian told France 3 television. "I hope that measures are taken to ensure Navalny's physical integrity, but also his freedom," he said.
"There is a major responsibility here for President (Vladimir) Putin."
Mr Navalny, Mr Putin's most prominent opponent, was arrested in January on returning to Russia after recovering from a near-fatal poisoning attack he says was orchestrated by the Kremlin. The Kremlin denies the allegation.
He is serving two and a half years on old embezzlement charges - which he says are politically motivated - in a penal colony in the town of Pokrov around 100 kilometres east of Moscow.
The 44-year-old began a hunger strike on 31 March to demand proper medical treatment for back pain and numbness in his legs and hands.
On Saturday, several doctors close to Mr Navalny warned that his health had rapidly deteriorated and he could "die any minute," demanding prison officials grant them immediate access.
"There is truly an authoritarian drift in Russia," Mr Le Drian said.
"Russia is responsible for Navalny's health, it must assume its role," he added, saying the European Union was closely following his fate.
"We have already taken action," he said, referring to EU sanctions.
"The package of sanctions is already significant, but there may be others."
Meanwhile allies of Mr Navalny have said they planned to stage what they have predicted would be the largest street protests in modern Russian history on Wednesday 21 April to draw attention to his deteriorating health.
US National Security Advisor Jake Sullivan has warned that Russia will face "consequences" if Mr Navalny dies.
"In terms of the specific measures we would undertake, we are looking at a variety of different costs that we would impose, and I'm not going to telegraph that publicly at this point, but we have communicated that there will be consequences if Mr Navalny dies," Mr Sullivan told CNN.
Germany has urged Russia to give Mr Nalvany adequate medical care.
"We urgently demand that Alexei Navalny receive adequate medical treatment and access to doctors he trusts. His right to medical care must be granted without delay," Foreign Minister Heiko Maas told Bild newspaper.
