President Donald Trump said Friday the US doesn't know whether Russian opposition leader Alexey Navalny was poisoned.
The US leader declined to accept the German government's assessment that the Russian opposition leader was attacked with a nerve agent at a White House news conference.
"I think it's sad, it's tragic. It's terrible. It shouldn't happen. We haven't had any proof yet. But I will take a look," Trump said.
NATO said on Friday that the poisoning was a "serious breach of international law and requires an international response".
But the military alliance's secretary-general, Jens Stoltenberg, said he did not wish to speculate on what action the response would entail.
He also called for "complete exposure" of Russia's alleged Novichok programme saying: "Russia has serious questions it must answer."
"The use of such a weapon is horrific," Stoltenberg said. "Those responsible for the attack must be bought to justice."
It came as MEPs called for the bloc to impose sanctions over the poisoning and push for an international investigation.
In a letter addressed to the European Union's top diplomat Josep Borrell and the Council Presidency, currently held by Germany, more than 100 MEPs call for the EU to "work towards an international investigation" to be conducted.
"We remain extremely sceptical that Russian authorities are fit and willing to investigate the real background of this crime.
"We all look back at a long history of (Russian President Vladimir) Putin opponents and independent journalists attacked and/or murdered and Russian authorities failing to bring to justice the real perpetrators," they added.
German authorities announced earlier this week that there was "unequivocal proof" that Navalny had been attacked with a Novichok nerve agent and that they would work with the EU and other international partners on "an appropriate joint reaction".
MEPs said the Navalny case reaffirmed "the necessity by the EU to swiftly establish the EU Human Rights Violations Sanction Mechanism, so we can hold accountable the people who are behind those attacks against oppositions figures and journalists".
"We cannot stand by and watch while opposition in Russia is systematically subjected to poison attacks," they went on.
Navalny fell ill during a flight from Siberia to Moscow on August 20 and his staff immediately suspected foul play. Doctors in Omsk, the Siberian city where he was initially treated before being transferred to Berlin's Charité Hospital, said however that Navalny suffered from "chronic pancreatitis" which had caused a glycemic imbalance.
He remains in intensive care.
Borrell said in a statement issued on Thursday evening that the EU "condemns in the strongest possible terms the assassination attempt on Alexei Navalny".
"The European Union calls for a joint international response and reserves the right to take appropriate action, including through restrictive measures," he went on, adding that the bloc also "calls on the Russian Federation to fully cooperate with the Organisation for the Prohibition of Chemical Weapons (OPCW) to ensure and impartial international investigation".
In a statement released earlier this week, the Russian embassy in Berlin said it had not yet received "any fact-based documents" from German authorities regarding the results of their investigation.
"We call on our partners to avoid any politicisation of this incident and to rely solely on credible facts, which we hope will be delivered as soon as possible," it added.