NATO Secretary-General Jens Stoltenberg has called on Russia to fully cooperate with an "impartial investigation" led by the Organization for the Prohibition of Chemical Weapons (OPCW) into the suspected poisoning of Russian opposition leader Alexei Navalny. 

Responding to a question from DW correspondent Teri Schultz in Brussels on Friday, Stoltenberg said NATO's response to the poisoning stems from the illegal use of a chemical weapon banned under international law and norms.

"It is not just an attack on an individual but it is also an attack on fundamental democratic rights," said Stoltenberg. "This is a blatant violation of international law and it requires an international response."

Earlier this week, the German government announced that Navalny had been poisoned with the military-grade nerve agent Novichok, the same chemical weapon British authorities say was used against Russian double agent Sergei Skripal and his daughter in 2018.

Read more: Navalny, Novichok and Nord Stream 2 — Germany stuck between a rock and a pipeline

Trump: US hasn't seen 'proof'

Speaking with reporters on Friday, US President Donald Trump said Washington did not have any evidence of Navalny's poisoning. "I don't know exactly what happened. I think it's tragic, it's terrible, it shouldn't happen," he said.

"We haven't had any proof yet, but I will take a look at it," Trump said at a White House press conference.

He said that he had no reason to doubt the evidence by German investigators, but added the US had not seen it. "Based on what Germany is saying that seems to be the case," Trump said. "I would be very angry if that's the case."

Earlier on Friday, US Deputy Secretary of State Stephen Biegun met with the Russian ambassador in Washington and urged Moscow to cooperate with an international probe into the attack.

The State Department official expressed "grave concern about German government findings that Russian opposition leader Alexei Navalny was poisoned by a chemical nerve agent," according to a spokesman.

'Completely unacceptable'

EU officials and German political leaders have proposed taking disciplinary action against Russia, including using sanctions.

"The use of chemical weapons is completely unacceptable under any circumstances, constitutes a serious breach of international law and international human rights standards," said EU foreign affairs chief Josep Borrell.

"The European Union calls for a joint international response and reserves the right to take appropriate actions, including through restrictive measures."

However, observers argue that the EU has little room to maneuver since the perpetrator remains unknown.

"Some say this is the biggest test for Europe's relations with Russia since the occupation of Crimea," said DW's chief political correspondent Melinda Crane.

Read more: Navalny poisoning: Confronting Russia, EU has limited options

International probe

Last month, Navalny was evacuated to Berlin after falling ill during a Russian flight. The hospital where he was treated in Siberia said he likely fell ill as a result of low blood sugar.

However, German doctors at the Charite Hospital in Berlin said tests showed he had been poisoned. Those claims were further backed by examinations conducted by the Bundeswehr, Germany's military.

German Chancellor Angela Merkel said that "beyond doubt" the poisoning was an attempted murder.

European officials have called for Russia to cooperate with the OPCW and submit to an independent probe.

"The OPCW continues to monitor the situation and stands ready to engage with and to assist any States Parties that may request its assistance," said OPCW Director-General Fernando Arias.

Read more: Opinion: Will Vladimir Putin ride out the Navalny storm?

ls/rc (AFP, dpa)