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