"The German government has announced that Alexei Navalny was the victim of an attack with a chemical nerve agent from the Novichok group. This is shocking, and I utterly condemn it," Stoltenberg said in a statement.

Berlin said that tests by its armed forces' chemical weapons laboratory found "unequivocal evidence" Navalny was poisoned with Novichok, the Soviet nerve agent also used to target a former spy in Britain in 2018.

"The use of a military-grade nerve agent makes it even more urgent that the Russian authorities conduct a full and transparent investigation," Stoltenberg said, insisting those responsible must be 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."

The 2018 Novichok poisoning of Sergei Skripal and his daughter in the English town of Salisbury sent already tense NATO ties with Russia into the deep freeze.