KYIV, June 20 (Reuters) - Ukraine's Deputy Defence Minister Hanna Maliar said the "biggest blow" in Kyiv's military campaign is yet to come, but admitted the operation is difficult as Russia throws all it can to stop the offensive.

"The ongoing operation has several objectives, and the military is fulfilling these tasks," Maliar said on the Telegram messaging app. "They are moving as they should have been moving. And the biggest blow is yet to come."

After months of acquiring Western weaponry, training and preparations, Ukraine began the first stage of its counteroffensive two weeks ago to reclaim the nearly fifth of its land now occupied by Russia.

"The enemy will not easily give up their positions, and we must prepare ourselves for a tough duel," Maliar said. "In fact, that is what is happening right now,"

The Ukrainian military, which had maintained strict silence about the campaign for more than a week, has since claimed small victories, saying on Monday that it has liberated several small settlements since.

Russian President Vladimir Putin said late last week the Ukrainian counteroffensive did not have any meaningful success, but officials and some Russian military bloggers say Kyiv has made small gains at the expense of huge troop and equipment losses.

It is impossible to independently verify the military operation along the most contentious points of the frontline, but Reuters was able to confirm that Ukraine's forces have been advancing in the early phase of the counteroffensive.

Maliar said that despite Ukraine's forces advancing in multiple directions in the south, Russia's forces push to advance in the east, concentrating its efforts there.

"Therefore, it is hot both in the east and in the south right now," Maliar said.