Mr Navalny, a Kremlin critic and corruption investigator, fell ill on a flight to Moscow and has been in an induced coma in a Berlin hospital since he was flown to Germany for treatment more than a week ago.
German authorities have said tests showed that he had been poisoned with a chemical nerve agent from the Novichok group.
Jens Stoltenberg, speaking after he chaired a meeting of NATO ambassadors, said: “There is proof beyond doubt that Mr Navalny was poisoned using a military-grade nerve agent from the Novichok group.
Germany briefed Allies on their specialists’ findings in the poisoning of Alexey Navalny. All #NATO Allies condemn this attack. We call on Russia to provide complete disclosure of the Novichok programme to @OPCW & to cooperate with an impartial, international investigation. pic.twitter.com/kUTh3ntrPe
— Jens Stoltenberg (@jensstoltenberg) September 4, 2020
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“The use of such a weapon is horrific.
“Any use of chemical weapons shows a total disrespect for human lives and is an unacceptable breach of international norms and rules. Nato allies agree that Russia now has serious questions it must answer.”
Mr Stoltenberg said Moscow must cooperate with the international chemical weapons organisation in “an impartial, international investigation” and provide information about its Novichok programme.
Asked about the case on Friday, US President Donald Trump told reporters that “I don’t know exactly what happened,” adding that “we haven’t had any proof yet.”
But later he said he would not be happy if Russia did poison Mr Navalny, “and that seems to be the case”.
We're working with our allies and the international community to hold those in Russia accountable wherever the evidence leads and restrict funds for their malign activities
White House press secretary Kayleigh McEnany said on Thursday that Mr Navalny was the victim of a “reprehensible” poisoning.
“Russia has used chemical nerve agents in the past,” she added, “and we’re working with our allies and the international community to hold those in Russia accountable wherever the evidence leads and restrict funds for their malign activities.”
Russian authorities have appeared reluctant to investigate what caused Mr Navalny’s condition, saying there had so far been no grounds for a criminal investigation.
Interior Minister Vladimir Kolokoltsev said a preliminary inquiry was ongoing, but added that he saw no signs of a crime in what happened to the most determined critic of president Vladimir Putin.
Mr Putin’s spokesman has brushed off allegations the Kremlin was involved in poisoning Mr Navalny and said Germany had not provided Moscow with any evidence about the politician’s condition.
“We have nothing to hide,” Russian Foreign Minister Sergey Lavrov said on Friday, asserting that German authorities had stonewalled Russian requests for information.
Mr Lavrov said the failure to provide information about Mr Navalny’s poisoning could indicate a lack of evidence.
“Our Western partners allow themselves to make arrogant demands in such a tone that suggests that they have nothing but pathos to put on the table,” he said.