MOSCOW — The Russian government said Tuesday that it was willing to launch a vigorous investigation into the recent sickening of a leading opposition figure, but only if it could be proved that he was poisoned.
On Monday German Chancellor Angela Merkel endorsed the conclusion of doctors at a Berlin hospital that the dissident, Alexei A. Navalny, had indeed been poisoned on a flight from Siberia, and called for an immediate investigation.
For the time being, though, that does not seem likely to happen.
“We don’t understand on what grounds our German colleagues are in such a hurry to use the word poison,” Russia’s presidential spokesman, Dmitri S. Peskov, told reporters on a conference call. “A substance has not been identified.”
Mr. Navalny, 44, who for almost a decade has challenged President Vladimir V. Putin politically and criticized his entourage for corruption, became one in a series of Kremlin opponents to collapse suddenly into a coma after drinking tea.
Doctors at the Siberian hospital that initially treated Mr. Navalny said laboratory results showed no signs of poisoning, while the hospital’s head doctor pointed to a metabolic disorder caused by low blood sugar as the most likely cause.
At the request of his family, Mr. Navalny, still in a coma, was evacuated to Germany by air ambulance on Saturday, and on Monday doctors at Charité Hospital in Berlin said he had been poisoned.
While they were not able to name the exact poison, the doctors said tests showed it came from a group of chemicals known as cholinesterase inhibitors, which interfere with the nervous system. They are used medically to treat Alzheimer’s and other types of dementia and in some forms are also found in chemical weapons and pesticides.
Ms. Merkel, in a statement issued Monday, said that “clinical findings indicate Aleksei Navalny was poisoned,” and that Russia should investigate.
It was a clear signal of support from the German leader for Russian opposition figures, defectors, journalists and human rights activists who have said for years that the Russian security services conduct secret operations to kill or incapacitate them with poison.
France on Tuesday echoed the German position. A Foreign Ministry statement expressed “serious concern” over the poisoning of Mr. Navalny, calling it a “criminal act perpetrated against a major actor of Russian political life.”
A Foreign Ministry spokesman said that Russia should undertake a “quick and transparent investigation,” and that “those responsible for this act need to be identified and brought to justice.”
Mr. Peskov, the Kremlin spokesman, said the German finding fell short of proof. Russian doctors, too, had detected low levels of cholinesterase in Mr. Navalny, he said, but ventured that this could have been caused by a variety of factors and not just by poison.
He noted the German doctors’ failure to identify a specific toxin. “The substance is absent,” he said. “Unfortunately, it cannot be found, and analyses do not show it.”
Mr. Navalny’s wife, spokeswoman and personal doctor had said Russian authorities endangered the opposition leader’s life by delaying his medical evacuation from Siberia for a day to allow time for the poison to metabolize, becoming more difficult to identify.
Mr. Peskov also denied a pattern of poisoning of Russian opposition figures.
“Here, I would not sketch out some tendency of murders, occurring in different countries of the world, of those who criticize the president of Russia,” he said. “This is not the case.”
Aurelien Breeden contributed reporting from Paris.