Russian Foreign Minister Sergei Lavrov says Moscow is going to be announcing retaliatory measures against top German and French officials in tit-for-tat sanctions sparked by the poisoning of outspoken Kremlin critic Aleksei Navalny with a nerve agent in August.

Navalny fell severely ill aboard a plane on his way from the Siberian city of Tomsk to Moscow, prompting the plane’s emergency landing in the city of Omsk. He was transferred later to the Charite clinic in Berlin, where doctors concluded that he had been poisoned with a nerve agent.

Last month, the European Union and Britain imposed asset freezes and travel bans against six senior Russian officials believed to be responsible for the “attempted assassination” of Navalny, as well as one entity involved in the program that has produced a group of military-grade nerve agents known as Novichok.

Lavrov also suggested that Navalny might have been poisoned with a nerve agent either in the Charite clinic in Berlin or aboard the plane that brought him from Omsk to the German capital.

Navalny, who remains in Germany receiving treatment, has blamed Russian President Vladimir Putin for ordering the poison attack, while the Kremlin has denied any involvement.