NATO allies have agreed that Russia must cooperate fully with an impartial investigation to be led by the Organisation for the Prohibition of Chemical Weapons into the poisoning of opposition leader Alexei Navalny, the alliance's chief said.
Germany, where Navalny is in hospital, has said he was poisoned with a Soviet-style Novichok nerve agent.
Russia has until now not opened a criminal investigation and said there is no evidence yet of a crime.
"Any use of chemical weapons shows a total disrespect for human lives, and is an unacceptable breach of international norms and rules," Secretary General Jens Stoltenberg told a news conference.
"NATO allies agree that Russia now has serious questions it must answer. The Russian government must fully cooperate with the Organisation for the Prohibition of Chemical Weapons (OPCW) on an impartial international investigation," he said, reporting back from a meeting of the alliance's ambassadors.
Mr Navalny is the most popular and prominent opponent of President Vladimir Putin, and the German announcement this week that he was poisoned by a nerve agent has raised the possibility of further Western sanctions against Moscow.
Mr Stoltenberg said the NATO allies called on Moscow to provide complete disclosure of the Novichok programme to the OPCW.
"Those responsible for this attack must be held accountable and brought to justice. Time and again we have seen opposition leaders and critics of the Russian regime attacked and their lives threatened. Some have even been killed," he said.
Describing the Navalny case as "an attack on fundamental democratic rights" as well as on an individual, he said NATO allies would continue to consult on the incident and "consider the implications".
Novichok was the same substance that was used against Russian ex-double agent Sergei Skripal and his daughter in the English town of Salisbury two years ago.
The Skripal case - the first offensive use of chemical weapons in Europe since World War II - prompted NATO to expel seven Russian diplomats in retaliation.
The EU's foreign policy chief Josep Borrell earlier called on Moscow to cooperate with an international probe into the poisoning and said the 27-nation bloc would not rule out sanctions.
Brussels condemned "in the strongest possible terms the assassination attempt", the Borrell statement said.
The EU said the use of chemical weapon "is completely unacceptable under any circumstances (and) constitutes a serious breach of international law and international human rights standards".
The Russian government "must do its utmost to investigate this crime thoroughly in full transparency and bring those responsible to justice. Impunity must not and will not be tolerated," Mr Borrell said.
The EU wants Russia to cooperate with the Organisation for the Prohibition of Chemical Weapons (OPCW), which is based in The Hague, to "ensure an impartial international investigation" needed to identify those responsible.
In Washington, Democratic senators pressed the US Treasury to impose sanctions on Russian organisations and individuals for interfering in the election, saying intelligence shows Moscow is seeking to damage Joe Biden's candidacy.
The Navalny poisoning is the latest in a long series of assassination attempts against Kremlin critics.
Russia denies there is any evidence that Mr Navalny was poisoned and Kremlin spokesman Dmitry Peskov said yesterday that Berlin had not provided Moscow with proof.
"There is no reason to accuse the Russian state," Mr Peskov said, rejecting talk of economic sanctions and urging the West not to "rush to judgement".
Already suffering from wide-ranging Western sanctions imposed over its 2014 annexation of Crimea, as well as the effects of the coronavirus pandemic and the drop in oil prices, Moscow will be anxious to avoid any further pressure on its economy.
Germany's announcement sent the ruble plunging to its lowest level against the euro since 2016 and Moscow's RTS stock exchange fell more than 3%.
Mr Navalny's top ally Ivan Zhdanov told AFP that the poisoning opened a "new chapter" in a Kremlin crackdown on dissent saying this is the first time a banned warfare agent was used against a top opposition leader on Russian soil.
Mr Navalny fell ill after boarding a plane in Siberia last month, with aides saying they suspect he drank a cup of spiked tea at the airport.
He was initially treated in a local hospital, where doctors said they were unable to find any toxic substances in his blood, before he was flown to Berlin for specialised treatment on 22 August.
The charismatic Yale-educated lawyer is still in the intensive care unit and remains on a ventilator.