KIEV, Ukraine—International observers reported Thursday that Russia-backed rebels moved large numbers of heavy weapons toward Ukrainian government positions hours before an outbreak of deadly fighting that threatened a delicate cease-fire.

While the observers didn't assign blame, the report from the Organization for Security and Cooperation in Europe gives credence to Kiev’s claims that the separatists started the Wednesday fighting, some of the worst since a cease-fire was signed in the Belarusian capital of Minsk in February....