One of the architects of the Moscow-backed separatist rebellion in eastern Ukraine eight years ago said Russia's invasion of Ukraine is a mistake, in comments that show the Kremlin cannot count on support from all pro-Russian opponents of Kyiv.

Alexei Alexandrov was one of the leaders of a movement in 2014 to reject Kyiv's rule and create an autonomous pro-Moscow territory in Ukraine's eastern Donbas region, triggering a war against Ukrainian government forces.

Russian President Vladimir Putin has said Russia attacked Ukraine last month in part to protect the separatist territory from Kyiv, though Western states say that is a pretext for an unprovoked land grab.

In an interview with Reuters last Friday, Alexandrov said...