Tens of thousands of Ukrainian soldiers currently fighting against the Russian forces in Ukraine have been trained at the hands of NATO, the alliance's Secretary-General, Jens Stoltenberg, said Sunday.

"We have to remember that NATO allies, like the United States, but also the United Kingdom and Canada and some others... they have trained Ukrainian troops for years," he revealed.

Stoltenberg also stressed that tens of thousands of Ukrainian troops trained by NATO allies were now "at the front fighting against invading Russian forces."

Additionally, the official told CNN that the Ukrainian armed forces were now better equipped, trained, and led than before due to NATO's interference.

NATO training Ukrainian soldiers is a reflection of the alliance's eastward expansion, which Russia is trying to curb through its operation in Ukraine that has been ongoing for over a month now.

Russia first entered Ukraine in response to calls from the people of Donbass, namely the Donetsk People's Republic and Lugansk People's Republic, who had asked Moscow to interfere due to Ukrainian shelling of the region and the killing of civilians.

Russia also stressed that it wanted to denazify and demilitarize Ukraine in light of the latter's actions.