Germany said Wednesday that tests performed on specimens taken from Russian opposition leader Alexei Navalny showed the presence of the Soviet-era nerve agent Novichok.
A special German military laboratory had shown proof of "a chemical nerve agent from the Novichok group," Chancellor Angela Merkel's spokesman, Steffen Seibert, said in a statement.
U.S. National Security Council spokesman John Ullyot said, "The United States is deeply troubled by the results released today (Wednesday)," and called Navalny's poisoning "completely reprehensible."
"We will work with allies and the international community to hold those in Russia accountable, wherever the evidence leads, and restrict funds for their malign activities," Ullyot added.
NATO Secretary-General Jens Stoltenberg also commented, saying, "This is shocking, and I utterly condemn it."
"The use of a military-grade nerve agent makes it even more urgent that the Russian authorities conduct a full and transparent investigation. Those responsible must be identified and held accountable. We will be consulting with Germany and all Allies on the implications of these findings. NATO regards any use of chemical weapons as a threat to international peace and security," Stoltenberg said.
Navalny fell ill on a flight from Siberia to Moscow on August 20 and, after an emergency landing, at first was taken to a hospital in the Siberian city of Omsk before later being flown for treatment and tests at a Berlin hospital.
The 44-year-old Navalny is one of Russian President Vladimir Putin's staunchest critics and an anti-corruption activist. The Kremlin has denied claims by Navalny's allies in Russia that authorities poisoned him, calling it "empty noise."
Russian doctors who treated Navalny in Siberia contested the German hospital's conclusion, saying they had ruled out poisoning and that their tests for poisonous substances came back negative.
Novichok is the same nerve agent that was used to poison former Russian spy Sergei Skripal and his daughter in Britain in 2018.
Seibert said the German government will inform the European Union and NATO about its Navalny test results and consult with them on "an appropriate joint response."