Berlin/Moscow: Kremlin critic Alexei Navalny, who is in intensive care in a Berlin hospital, was poisoned with a Soviet-style Novichok nerve agent in an attempt to murder him, German Chancellor Angela Merkel said on Wednesday.

She said Berlin now expected Moscow to explain itself and that Germany would consult its NATO allies about how to respond, raising the prospect of new Western sanctions on Russia.

Moscow has denied involvement in the incident. The Russian foreign ministry said Germany's assertion was not backed by evidence and complained about the way Germany had chosen to release information about Navalny.

"This is disturbing information about the attempted murder through poisoning against a leading Russian opposition figure," Merkel said. "Alexei Navalny was the victim of an attack with a chemical nerve agent of the Novichok group. This poison could be identified unequivocally in tests."

Novichok is the same substance that Britain said was used against a Russian defector and his daughter in an attack in England in 2018.

Navalny, 44, is an outspoken opponent of Russian President Vladimir Putin and has specialised in high-impact investigations into official corruption. He was airlifted to Germany last month after collapsing on a domestic Russian flight after drinking a cup of tea his allies said was poisoned.

The Kremlin said it wanted a full exchange of information and that Germany and Russia should cooperate, but said it was unable yet to give a proper statement about the German findings.

Russian authorities and doctors have previously said they could find no evidence Navalny was poisoned.

Russia is already under Western sanctions after its annexation of Crimea from Ukraine six years ago. Another stand-off with European nations or the United States may hurt its economy further. The Russian rouble extended losses against the euro after the German government statement.

Navalny in a serious condition

Berlin's Charite hospital, which is treating Navalny, said he remained in a serious condition in an intensive care unit connected to an artificial lung ventilator even though some of his symptoms were receding.

It said it could not rule out long-term consequences from his poisoning and that it expected him to go through a long period of illness.

Lithuanian Foreign Minister Linas Linkevicius said in a tweet that the German government's identification of the Novichok poison indicated Russian authorities were behind Navalny's poisoning.

The Kremlin has rejected any suggestion that it or the Russian state was involved.

Allies of Navalny echoed Linkevicius' view. Leonid Volkov, a Navalny aide, said on Twitter that the use of Novichok was like leaving Putin's signature at the scene of the crime.

Ivan Zhdanov, another close Navalny ally, said on Twitter that Novichok could be administered only by Russia's intelligence agencies.

German Foreign Minister Heiko Maas urged Russia to investigate Navalny's poisoning now that clinical tests had shown he had been attacked with a chemical nerve agent.

"This makes it all the more urgent that those responsible in Russia be identified and held accountable," Maas told reporters. "We condemn this attack in the strongest terms."

Novichok is a deadly group of nerve agents developed by the Soviet military in the 1970s and 1980s.

Steffen Seibert, a German government spokesman, had earlier said in an emailed statement that tests conducted at a German military laboratory had produced "unequivocal evidence" that Novichok had been used.

Britain says Russia used Novichok to poison former spy Sergei Skripal and his daughter in the British city of Salisbury in 2018. Russia has repeatedly denied any involvement in the attack, which the Skripals survived. A member of the public, 44-year-old Dawn Sturgess, was killed.