Russia has alleged that Kremlin critic Alexei Navalny could have been poisoned in Germany or on the medical plane to Berlin.
Moscow has also announced planned retaliatory sanctions against France and Germany over the alleged poisoning.
Navalny, a prominent Russian opposition figure, fell ill on a flight in Siberia on August 20 and was airlifted to Berlin for treatment two days later.
The global Organisation for the Prohibition of Chemical Weapons has confirmed that a Novichok nerve agent was present in Navalny's blood samples.
Navalny received treatment in the Charite hospital in Berlin for 32 days before being discharged in September.
The poisoning has worsened relations between Russia and western Europe.
The EU, led by France and Germany, unveiled sanctions against six prominent figures close to Russian President Vladimir Putin in October.
Russian Foreign Minister Sergei Lavrov announced that employees of the French and German leaders will also be targeted by measures in response.
"Since the (European) sanctions are aimed at officials of the Russian presidential administration, our retaliatory sanctions will reflect them," Lavrov told an online news conference on Thursday.
"They have been decided, and we will soon inform our French and German colleagues."
Lavrov specified that the sanctions were aimed at "senior executives of the German and French leadership apparatus".
Moscow has rejected any responsibility in Navalny's poisoning and has repeatedly claimed that any allegations are a plot to harm Russia.
"We have reason to assume that whatever happened to [Navalny] concerning the toxic substance in his body could have happened to him in Germany or on the plane where he was loaded and sent to the Charite clinic," said Sergei Lavrov.
The foreign minister gave no further evidence or details to this claim.
Russian authorities said Navalny had no poison in his system when he was hospitalised in Siberia.
President Vladimir Putin has stated that he personally intervened to allow Navalny to be evacuated to Berlin.
Navalny, who is still recovering, has directly accused Putin of being behind his poisoning.
Last week, the Moscow offices of Alexei Navalny's anti-corruption foundation were searched in connection with a claim for unpaid damages.