KAMENSK-SHAKHTINSKY, Russia — A Russian aid convoy rumbled toward rebel-held territory in eastern Ukraine on Thursday, stirring fears of a direct confrontation between Russia and Ukraine after months of hands-off sparring through armed proxies between the two bitterly estranged neighbors.

Warning that Ukraine would deploy “all forces available” to halt the convoy if it refused to submit to inspection by Ukrainian border guards, Ukraine’s military spokesman, Andriy Lysenko, said that “any attempt to introduce any convoy without the permission of Ukraine would be considered an act of open aggression.”

The convoy of around 260 trucks, mostly military vehicles that had been recently spray-painted white and covered with white tarpaulins, came within miles of the Ukrainian border on Thursday but then halted its advance, turning onto a long dusty road near a Russian military base on the outskirts of the Russian town of Kamensk-Shakhtinsky.

Ukraine and its Western allies have cast a mistrustful eye on the Russian mission from the start, regarding it more as a cynical ploy to help beleaguered rebel forces stave off defeat and thus extend a war that they say the Kremlin itself has covertly stoked, and is the primary cause of the humanitarian crisis that the aid convoy is supposed to relieve.