Russia has shifted most of the focus of its war to eastern Ukraine, following a series of setbacks especially around the capital Kiev.

That push in the area known as the Donbas could signal a protracted conflict, the US says.

What would Vladimir Putin need before he could claim his goal of "liberating" Ukraine's old industrial heart, and is that possible?

Russian forces have already caused a humanitarian catastrophe in the east, turning Mariupol into ruins, but they have failed to inflict losses on the Ukrainian army.

Announcing a new attack in the east, President Volodymyr Zelensky vowed "We will fight for every meter of our land," the

reports .

Ukraine had already deployed its most trained forces in the east due to an eight-year war with Russian-backed separatists.

They are thought to have suffered heavy losses, but are still a significant challenge to the invading Russian army.

Why is Donbas important for Ukraine?

When President Putin talks about Donbas, he is referring to Ukraine's old coal and steel production area.

But he really means the entirety of the two major eastern regions, Luhansk and Donetsk, which run from outside Mariupol in the south to the northern border.

"The main issue is that the Kremlin has been identified as a Russian-speaking part of Ukraine that is more Russian than Ukraine," said Sam Cranny-Evans of the Royal United Services Institute.

These areas may be widely Russian-speaking, but they are no longer pro-Russian.

"Mariupol was one of the most pro-Russian cities in Ukraine, but they are no longer supporting Russia's actions," said defense expert Konrad Muzyka, head of Rochan Consulting.

Last week, Russia claimed to have taken control of 93 percent of the Luhansk region and 54 percent of Donetsk.

Russian forces are trying to encircle the Ukrainian army in the east.

They have taken control of Izyum - a strategic town on the road between Ukraine's second city Kharkiv and separatist areas - and bombed a number of Luhansk cities, including Rubizhne, Lysychansk, Popasna and Severodonetsk, destroying apartment blocks and killing houses. even civilians.

Cities that are now in Russian sights have already experienced years of war since 2014 and now barely passes a day without a deadly attack.

Civilians are fleeing ahead of Russian progress.

Local leader Serhiy Haidai said 20 children were taken to safety from the basement of a kindergarten in Lysychansk on Thursday and about 200 civilians boarded buses in Severodonetsk.

They were leaving for western Ukraine with nothing, Haidai said, but they were alive.

Maryna Agafonova, 27, left her family home in Lysychansk earlier this week.

At the start of the war she said Russian forces had targeted the outskirts of the city, but had recently hit the center.

"They attacked hospitals and residential buildings.

There is no heat or electricity.

"My parents are still there," he said.

"Ukrainian forces were still there in numbers," she told the BBC. "They will not let the Russians invade it."

Russia's next goal will be to push south to encircle Slovyansk, a town of 125,000 that had already experienced close combat when captured by Russian-backed forces in 2014 before retaking it.

Why does Putin want to control Donbas?

The Russian leader has repeatedly made baseless allegations that Ukraine committed genocide in the east.

When the war broke out, two-thirds of the eastern regions were in Ukrainian hands.

The rest were led by separatists, who created Russian-backed states during a war that began eight years ago.

If Russia were to invade the two major regions, then the next step could be their annexation, as it did with Crimea in 2014.

Before invading Ukraine, Putin recognized the entirety of the two eastern regions as independent of Ukraine.

The "Russian puppet" leader in Luhansk has already spoken of a referendum in the "near future", although the idea that even a fake vote could take place in a war zone seems absurd.

Life under Russian-backed separatist control is calmer, although separatist authorities have accused Ukrainian forces of shelling apartment buildings and killing civilians.

Officials in Donetsk state say 72 civilians have died since mid-February.

Will the Ukrainian forces endure?

At the beginning of the war, the 10 brigades that made up Operation Joint Forces (JFO) in the east were considered the most equipped and trained soldiers Ukraine has.

"We do not really know the strength of the Ukrainian forces," said Sam Cranny-Evans of Russia, who believes the number of volunteers will increase in recent weeks.

Russian forces have already suffered heavy losses after more than five weeks of conflict and morale is thought to be low.

They consist of men registered from local separatist areas as well as the wider Russian army.

However, they have captured a significant part of the southeast and will hope to control the entire stretch of coastal territory from Crimea to the Russian border.

"The main goal for the Ukrainians is to suffer the greatest possible losses on the Russian side, and the Ukrainians are using asymmetric tactics to avoid major battles," said Konrad Muzyka.

Mykyta, who managed to escape the Russian bombing of Mariupol, said he was confident the Ukrainian army would succeed in fighting back.

"One day they will return our cities, the Azov battalion will not hand over Mariupol," he told the BBC.

"The Ukrainian army is very cunning, I have not seen them in my city, but I have heard them, they are very good at disguise," he concluded.