The poisoning of Alexei Navalny will be discussed at an urgent NATO meeting after European leaders threatened fresh sanctions against Moscow.
Putin’s administration has strongly denied poisoning the opposition leader.
The 44-year-old lawyer fell ill on a flight last month and was evacuated to Germany for treatment.
Western leaders have demanded answers from the Kremlin after Berlin said there was “unequivocal evidence” that he had been afflicted by the Soviet-era nerve agent Novichok.