Russian forces on Tuesday stepped up their offensive on the last pocket of resistance around Lugansk in Ukraine's eastern Donbas region, as the conflict entered its fourth month, according to AFP.

Since Moscow's military operation in late February, Western support has helped Ukraine hold off its neighbor's advances in many areas, including the capital Kyiv. Russia is now focused on securing and expanding its gains in Donbas and the southern coast.

"The coming weeks of the war will be difficult, and we must be aware of that," Ukraine's President Volodymyr Zelensky said in his nightly address Monday after regional leaders and residents reported heavy bombardments.

"The most difficult fighting situation today is in Donbas," Zelensky said, singling out the worst-hit towns of Bakhmut, Popasna and Severodonetsk.

Ukraine's armed forces said Russian troops were conducting non-stop "offensive operations" in the region.