Stepan Poltorak also accused Russia of keeping several thousand troops in the war-torn region and said his impoverished country was in dire need of Western weapons to help quash the two-year revolt.
"In my opinion, it will take years," Poltorak told a small group of reporters when asked how long it may take to resolve the war.
"As for the need for lethal weapons, of course they are always needed," he added.
The United States and the European Union back Kiev's claims of Russia taking an active part in a war that began two months after the February 2014 ouster of Ukraine's Moscow-backed president.
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The White House refers to the presence of "combined Russian-separatist forces" in Ukraine's eastern industrial regions of Lugansk and Donetsk.
The European Union has also sanctioned Moscow military officials whom it accused of "being involved in supporting the deployment of Russian troops in Ukraine."
But Washington and EU nations have only provided Ukraine with support equipment such as advanced radar out of apparent fear that arming its forces may only provoke Russian President Vladimir Putin.
Poltorak estimated the number of combined rebel and Russian soldiers fighting government forces at more than 40,000.
"There are slightly over 7,000 Russian troops. They are being rotated all the time."
He said the insurgents would continue staging attacks aimed at provoking retaliatory strikes that would make Kiev look like "the side that is initiating violations of the Minsk (peace) agreements" signed in February 2015.
That deal helped stem the worst of the violence but failed to find a workable solution that would settle the separatist regions' status with a unified Ukraine.
Ukraine believes Russia annexed Crimea in March 2014 and plotted the eastern conflict in order to keep its western neighbour in its sphere of influence and keep it from seeking membership in the European Union and the NATO military bloc.