Ukrainian forces have launched a counteroffensive near the Russian-held town of Izium in eastern Ukraine, a regional governor said on Saturday, in what could prove a serious setback for Moscow's plans to capture the entire Donbas region.

Russian forces have focused much of their firepower on the Donbas in a "second phase" of their incursion announced on April 19, after they failed to reach the capital Kyiv from the north in the early weeks of the war.

But Ukraine has been retaking territory in its northeast, driving the Russians away from the second-largest Ukrainian city of Kharkiv. Keeping up pressure on Izium and Russian supply lines will make it harder for Moscow to encircle battle-hardened Ukrainian troops on the eastern front in the Donbas.

Ukraine's President Volodymyr Zelenskiy also said complex talks were under way to find a way to evacuate a large number of wounded soldiers from a besieged steel works in the port of Mariupol in return for the release of Russian prisoners of war.

Mariupol, which has seen the heaviest fighting in nearly three months of war, is now in Russian hands but hundreds of Ukrainian defenders are still holding out at the Azovstal steelworks despite weeks of heavy Russian bombardment.

Commenting on latest developments in eastern Ukraine, regional governor Oleh Sinegubov said in comments aired on social media: "The hottest spot remains the Izium direction."

"Our armed forces have switched to a counteroffensive there. The enemy is retreating on some fronts and this is the result of the character of our armed forces," he said.

Russia's Foreign Minister Sergei Lavrov said on Saturday the West's sanctions amounted to a "total hybrid war" against Moscow and that it was hard to predict how long it might last. (Reuters)