BERLIN (AP) — Germany confirmed Friday that Russian opposition leader Alexei Navalny has been questioned as a witness by German investigators this week about his poisoning in Russia four months ago.

Navalny fell sick on Aug. 20 during a domestic flight in Russia and was flown while still in a coma to Berlin for treatment two days later. Labs in Germany, France and Sweden, and tests by the Organization for the Prohibition of Chemical Weapons, established that he was exposed to a Soviet-era Novichok nerve agent.

The case has sparked further tension between Berlin and Moscow, which denies any official involvement in the poisoning and has accused Germany of dragging its feet in providing judicial assistance to Russian investigators.

A spokesman for Germany's Justice Ministry said that as part of a request from Russia, Berlin prosecutors questioned Navalny on Thursday “as a victim witness.” Navalny himself had written about the questioning on social media.

The ministry spokesman, Marius Leber, said no Russian investigators were present during the questioning.

“Mr. Navalny was poisoned in Russia, and he expressly objected to the presence of Russian investigators during his questioning,” said Leber. “This procedure is in accordance with the European Convention on Mutual Assistance in Criminal Matters.”

No decision has been made yet on Russia's requests for judicial assistance, including on whether to transmit the results of Navalny's questioning to Moscow.

Germany has called for Russia to conduct credible investigations of its own into the poisoning.

“All the necessary information, such as blood and tissue samples and clothing, are present in Russia,” Leber said.

Russian President Vladimir Putin on Thursday again rejected allegations that the Kremlin was behind the poisoning of his top political foe, saying that “if there was such a desire, it would have been done.”

Earlier this week, the investigative group Bellingcat and Russian news outlet The Insider named supposed Federal Security Service operatives they alleged had trailed Navalny.