Ukraine's position "worsened a little" in the eastern Ukrainian city of Sievierodonetsk, but Kyiv's forces defended their positions against Russia in an industrial zone as heavy fighting raged, the regional governor said on Monday.

Ukrainian forces had been trying to hold their ground after saying they had retaken half of the embattled city in Luhansk province, where Russian forces have concentrated their assault on Ukraine's eastern Donbas heartland.

"The fiercest fighting is in Sievierodonetsk. Fast-moving fighting is happening right now," Luhansk regional governor Serhiy Gaidai told national television.

"Our defenders managed to undertake a counter-attack for a certain time; they liberated almost half of the city. But now the situation has worsened a little for us again," he said. (Reporting by Natalia Zinets Writing by Conor Humphries/Tom Balmforth Editing by David Goodman)