Ukraine landed special forces to fight in embattled parts of the eastern city of Pokrovsk earlier this week, just as Russia said it had surrounded Kyiv's forces in the area, two Ukrainian military sources said Friday.

The operation proved how Ukraine is battling to stabilize the situation in the strategically important city after scores of Russian troops breached its perimeter this month. Russia's capture of Pokrovsk, an important road and rail hub, could enable further advances into the eastern Donetsk region, which Russia aims to fully occupy. Moscow's military has been inching forward toward Pokrovsk for more than a year.

The Ukrainian special forces landed in a Black Hawk helicopter a few days ago in the operation, which was complicated by Russian drone activity, a source in the 7th Rapid Response Corps said.

Military spy chief Kyrylo Budanov oversaw the operation, and the troops headed to areas of the city claimed by Russia and seen by Moscow as vital for Ukrainian supply lines, the other source said.

At least 10 servicemen could be seen dismounting from a helicopter in a field in a video seen by Reuters, which could not independently confirm the location or date when the video was filmed.

Russia's defense ministry did not immediately respond to a request for comment about the Ukrainian operation.

At least 200 Russian troops have penetrated the city's defenses and made it inside, the Ukrainian military has said.

Kyiv's army chief acknowledged Thursday that the situation was "difficult," saying supply lines and defenses in the area needed to be bolstered.

DeepState, a Ukrainian open-source mapping project, estimated that at least half of the city lies in a contested area fully controlled by neither side.

Ukrainian President Volodymyr Zelenskyy referred directly to the battle of Pokrovsk in his evening address.

"We continue to destroy the occupier," he said. "The most important thing is to stop Russian attacks wherever possible."