What happens next in the Russia-Ukraine war in Donbas, east Ukraine?

Russia’s new offensive in the Donbas region could prove to be extremely significant and decisive in the war, analysts warn, and could determine how the country’s territorial boundaries look in weeks and years to come.

“The Russian war machine in the east could prove to be a very painful threat for Ukraine quickly,” Maximilian Hess, fellow at the Foreign Policy Research Institute, told CNBC Tuesday.

“It’s quite clear that Russia’s war aims remain quite extensive,” Hess added, saying that how the battle for Donbas proceeds “will determine how much of Ukraine east of the Dnipro (a river that bisects Ukraine) that [Russian President Vladimir] Putin carves away.”

“I think it’s pretty clear annexation is Putin’s long term goal, how much annexation is the question,” Hess added.

Russian officials have stated that their main objectives in this new phase of the war is the “complete liberation” of the two breakaway, Russian-backed “People’s Republics” of Luhansk and Donetsk. But most analysts believe that the wider Donbas region, an industrialized area rich in coal reserves, will be annexed by Russia.

Moscow has fomented separatist sentiment in the region over the last eight years ever since it annexed Crimea in 2014, although it denies backing the region’s rebels.

Russia’s long-anticipated offensive in the east appeared to begin in earnest on Monday with its military forces unleashing attacks on a number of areas, with Ukrainian President Volodymyr Zelenskyy saying that “the battle for Donbas” had begun.

By Tuesday morning, Russia’s defense ministry claimed to have struck more than 1,200 targets in Ukraine overnight and later that day, there were numerous reports of intensifying rocket and artillery fire in eastern Ukraine. Officials said Russian forces have seized control of Kreminna, a city in the Luhansk region where street battles reportedly took place.

Wednesday morning, the U.K.’s defense ministry said in an intelligence update that Ukrainian forces were repelling “numerous attempted advances” by Russia in the eastern Donbas region.

The re-focusing on eastern Ukraine comes after Russia pulled back many of its troops from areas around the capital Kyiv and other northern parts of the country after failing to make military gains there. The Pentagon believes that Russia has significantly increased its fighting power in eastern and southern Ukraine now, however, with more battalion tactical groups moved to the area last weekend.