Russia pummeled towns and villages in Ukraine's northeastern Kharkiv region on Monday, days after launching a surprise ground offensive over the border that has forced thousands to evacuate.

The Ukrainian army acknowledged Russia was "achieving tactical success" after launching a ground assault on Friday that has sparked the evacuation of almost 6,000 people.

But President Volodymyr Zelenskyy stressed Ukraine had sent in reinforcements, and that a "counterattack" was ongoing.