Ukraine's top military commander has admitted his troops are facing a "difficult situation" defending the key eastern front city of Pokrovskoye from a large Russian force.

Gen. Oleksandr Sierskiy said Ukrainian troops were facing "several thousand of the enemy," denying Russian claims that they were surrounded or had no way out.

The commander confirmed that Ukrainian special forces have been deployed to protect a key supply line that sources say is under Russian fire.

Moscow's Defense Ministry said Ukrainian troops were surrendering and that 11 special forces members were killed after arriving by helicopter.

Kiev has denied the claim.

On Saturday, the commander-in-chief said in a Telegram message that he had "returned to the front" to receive a report in person from military commanders in the eastern Donetsk region.

The Russian Defense Ministry announced that Ukrainian military security forces had returned fire while deploying to the northwest of the city, killing 11 soldiers.