DONETSK, Ukraine — Separatist rebels on Monday attacked a caravan of cars carrying refugees trying to flee war-ravaged eastern Ukraine, killing “dozens” of people in a devastating barrage of artillery fire, Ukrainian military officials said, though rebel leaders denied there had been any attack at all.

Col. Andriy Lysenko, a spokesman for the Ukrainian military, said at a briefing in Kiev, the capital, that “terrorists had perpetrated a bloody crime” by attacking the refugee convoy with Grad rocket systems and other heavy weapons supplied by Russia.

Although Colonel Lysenko did not give a precise tally of the fatalities, if confirmed, the episode would easily rank among the most deadly for civilians since separatists began seizing cities and towns in the region more than four months ago.

“Many people died, including women and children,” he said, adding that the assault took place on the main highway leading south out of the regional capital of Luhansk, between the villages of Khryashchuvatye and Novosvitlivka.