Ukraine President Volodymyr Zelensky said Thursday that Russia controls about one-fifth of his country, including ground gained over Moscow's invasion, the annexed Crimean peninsula and territory held by Moscow-backed separatists.

"Today, about 20 percent of our territory is under the control of the occupiers," the Ukrainian leader said during an address to lawmakers in Luxembourg.

Russian forces are solidifying their hold on the eastern Donbas region and pushing steadily towards Ukraine's de facto administrative centre in that region, Kramatorsk.

However, they pulled back from regions around the capital and in the northeast to focus on their offensive for the eastern industrial region.

Zelensky said that in 2014, Kremlin-backed separatists and the Russian military controlled 43,000 square kilometres (16,600 square miles), an area he compared to the size of the Netherlands.

But that figure -- more than three months into Russia's invasion -- had increased to nearly 125,000 square kilometres, territory he said was "much greater" than the Netherlands, Belgium and Luxembourg combined.

He also told lawmakers that an area more than twice that size -- "nearly 300,000 square kilometres" -- had been "polluted" with mines and unexploded ordnance.

"Twelve million Ukrainians are displaced and more than five million have gone abroad," he added.