The poisoning of Alexei Navalny will be discussed at an urgent NATO meeting on Friday after European leaders threatened fresh sanctions against Moscow, which has furiously denied targeting the Russian opposition leader.

The 44-year-old lawyer, one of President Vladimir Putin's chief critics, fell ill on a flight last month and was evacuated to Germany for treatment.

Western leaders have demanded answers from the Kremlin after Berlin said there was "unequivocal evidence" that he had been afflicted by the infamous Soviet-era nerve agent Novichok.

The same substance was used against Russian ex-double agent Sergei Skripal and his daughter in the English town of Salisbury two years ago, and Germany's claim prompted widespread condemnation.

Germany will brief a special meeting of the North Atlantic Treaty Organization, with the alliance's Secretary General Jens Stoltenberg to hold a press conference after it concludes.

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 weapons "is completely unacceptable under any circumstances (and) constitutes a serious breach of international law and international human rights standards."

'Do its utmost' 

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," 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 a letter to Borrell, more than one hundred MPs from the European Parliament called for the UN and the Council of Europe to also be involved.  We are "extremely skeptical that the Russian authorities are able and willing to investigate the true background to this crime," they said.

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 Navalny was poisoned and Kremlin spokesman Dmitry Peskov said Thursday that Berlin had not provided Moscow with proof.

"There is no reason to accuse the Russian state," Peskov said, rejecting talk of economic sanctions and urging the West not to "rush to judgement".

'Poisoned relations with West'  

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 three percent.

"Russia's relations with the West have once again been poisoned by Novichok," wrote business daily Kommersant. "The main question is, how far will they decide to go?" it said.

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.

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 August 22.

A toxicologist in Siberia on Friday said Navalny's health could have deteriorated due to dieting, stress or fatigue, insisting no poison had been found in samples. The charismatic Yale-educated lawyer is still in the intensive care unit and remains on a ventilator.