The Russian military expected Kyiv to fall within 12 hours of the February 24 invasion, Ukrainian Defense Minister Oleksiy Reznikov told CNN's Matthew Chance in an exclusive interview in Brussels.

A document was found on a Russian military officer who was killed in the invasion, which stated the Russian military objectives, Reznikov said.

"They thought they were going to be in the center of Kyiv in 12 hours," he said the document stated.

Russians naively anticipated that within 72 hours of their invasion, the Ukrainian government would flee the capital, Reznikov said.

"Frankly speaking, our partners in the different capitals of the world also were naive. They told us that invasion was imminent, and you will fall. You only have 72 hours. That's why they didn't give us heavy weaponry."

Only after Ukraine liberated the Kyiv region in late March, did Western partners start to provide Ukraine with heavier weapons, he said.