Russia assaulted cities and towns along a boomerang-shaped front hundreds of miles long and poured more troops into Ukraine on Tuesday in a potentially pivotal battle for control of the country’s eastern industrial heartland of coal mines and factories.

If successful, the Russian offensive in what is known as the Donbas would essentially slice Ukraine in two and give President Vladimir Putin a badly needed victory following the failed attempt by Moscow’s forces to storm the capital, Kyiv, and heavier-than-expected casualties nearly two months into the war.

The eastern cities of Kharkiv and Kramatorsk came under deadly attack. Russia also said it struck areas around Zaporizhzhia and Dnipro west of the Donbas with missiles. Multiple explosions were heard early Wednesday in the southern city of Mykolaiv, the regional governor said. A hospital was reported shelled earlier in the nearby town of Bashtanka.

In Mariupol, the now-devastated port city in the Donbas, Ukrainian troops said the Russian military dropped heavy bombs to flatten what was left of a sprawling steel plant and hit a hospital where hundreds were staying.