Russia has moved most of the focus of its war to eastern Ukraine, after pulling back its forces from near the capital Kyiv. Ukraine's leader Volodymyr Zelensky says the battle for the area known as Donbas has begun and a very large part of Russia's army is taking part.

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

Ukraine's best-trained forces were already posted in the east because of an eight-year war with Russian-backed separatists. They are thought to have suffered heavy losses, but are still a significant challenge to Russia's invading army.

Russian forces have already triggered a humanitarian catastrophe in the east, but are still not in control of the port city of Mariupol. Ukraine's president has said "we will fight for every metre of our land" and Russia's new offensive could herald a protracted conflict.

When President Putin talks of Donbas, he is referring to Ukraine's old coal and steel-producing area. What he really means is the entirety of two big eastern regions, Luhansk and Donetsk, which run from outside Mariupol in the south all the way to the northern border.

"The key is that it has been identified by the Kremlin as a Russian-speaking part of Ukraine that is more Russia than Ukraine," says Sam Cranny-Evans of the Royal United Services Institute.

These areas may be broadly Russian-speaking, but they are no longer pro-Russian. "Mariupol was one of the most pro-Russian cities in Ukraine and to level it is beyond my comprehension," says defence specialist Konrad Muzyka, head of Rochan Consulting.

A month into the war, Russia claimed to have taken control of 93% of Luhansk region and 54% of Donetsk. Russia's president is still a long way from subduing the entire area, but even if he were to be able to claim victory, this is a very large territory to control.

The Russian leader has repeatedly made the unfounded accusation that Ukraine has carried out genocide in the east.

When the war began, almost two-thirds of the eastern regions were in Ukrainian hands. The rest was run by Russian proxies, who created Moscow-backed statelets during a war that began eight years ago.

Just before the war, President Putin recognised all of the two eastern regions as independent of Ukraine.

If Russia were to conquer both big regions, it would give Vladimir Putin some sort of achievement from Russia's war. The next step would then be to annexe Donbas, just like he did with Crimea after a discredited referendum in 2014.

And if it came before 9 May, he would even be able to celebrate on Victory Day, when Russia's military still marks the defeat of Nazi Germany in 1945.

Russia's puppet leader in Luhansk has already spoken of having a referendum in the "near future", although the idea of even a sham vote in a war zone seems absurd.

Ukraine's military believes he wants full control not just of Donbas but also of the southern region of Kherson, to the north and west of Crimea. That would give him a land bridge along the south coast to the Russian border and control of the water supply to Crimea.

Much of Donetsk and key areas of Luhansk are still in Ukrainian hands, so Russian forces are trying to encircle Ukraine's army in the east, moving in from the north, east and south. "This is a big territory to control, and I think we shouldn't underestimate the geographical complexities of this," says Tracey German, professor in conflict and security at King's College London.

They have so far failed to capture Ukraine's second city of Kharkiv south of the Russian border, but they have seized control of Izyum - a strategic town further down the main motorway heading into the separatist-controlled east.

"If you look at what they're doing around Izyum, it's following the main lines of the highways and that would make sense, given that they're moving most of their equipment by road and rail," says Prof German.

The towns now in Russian sights have already experienced years of war since Russian-backed separatists first seized large parts of Donbas.

The next big target down the M03 road is Slovyansk, a city of 125,000 people that was seized by Russian-backed forces in 2014 before being recaptured. Capturing Kramatorsk, just to the south, is also a big objective.

The US-based Institute for the Study of War (ISW) says if Ukraine holds on to Slovyansk, Russia's campaign to seize both regions "will likely fail".

Russian forces have attacked a succession of Ukrainian-controlled towns in Luhansk as they advance from the north and east. They have entered Kreminna, to the north-west of Severodonetsk and have bombarded towns including Rubizhne, Popasna and Lysychansk.

These towns are important because controlling them would enable Russia to drive west and link up with Russian forces pushing south-east of Izyum, says the ISW. That is why Ukraine's military is concentrating its counterattacks in that area outside Izyum.

The Russians need to control the supply lines by road and block Ukrainian access to rail routes from the west. Rail is the most effective transport for Ukrainian troops and heavy weaponry and it is the fastest route out for fleeing civilians.

Controlling parts of the rail network would also enable Russian forces to move their troops and supplies.

Regional military administration head Serhiy Haidai believes Russia's goal is to destroy everything blocking their way in order to achieve Vladimir Putin's target of reaching Luhansk's western regional borders.

Maryna Agafonova, 27, fled her family's home in Lysychansk, leaving her parents behind as Russian shells continued to fall. "They attacked hospitals and residential buildings. There is no heating and no electricity."

Civilians are being moved out ahead of the Russian advance. "It's far more scary to remain and burn in your sleep from a Russian shell," warned Mr Haidai.

Train lines have been cut to Izyum in the north and to Mariupol and Melitopol in the south. Services have been disrupted from Slovyansk and no trains have run from the nearby hub of Kramatorsk since a rocket attack killed 57 people as they waited to board trains.

At the start of the war, the 10 brigades that made up Ukraine's Joint Forces Operation (JFO) in the east were regarded as the best-equipped and best-trained soldiers that Ukraine has.

"We don't really know the strength of Ukrainian forces now," said Sam Cranny-Evans of Rusi, who believes their numbers will have been boosted by volunteers in recent weeks.

"The main goal for the Ukrainians is to inflict as big losses on the Russian side as possible and the Ukrainians are using asymmetric tactics to avoid big battles," says Konrad Muzyka.

US officials say Russia now has 76 battalion tactical groups in the east, having added 11 to the area in recent days, and more being resupplied north of the border. They typically number 700 to 900 men.

Despite their reinforcements, there is widespread scepticism that Russian forces will achieve any breakthrough fast. ISW believes they may be able to wear down Ukrainian positions but likely at high cost, while Ukrainian military experts believe many of Russia's battalion groups are in poor condition.

Russian forces have already sustained high losses after seven weeks of conflict and morale is thought to be low. Their units are made up of men enlisted from the local separatist areas, many under duress, as well as the broader Russian army.

Life under the control of Russian-backed separatists is quieter, although separatist authorities have accused Ukrainian forces of shelling residential buildings and killing civilians. Officials in the Donetsk statelet say 72 civilians have died since mid-February. Far more have been killed in Russian attacks on Ukrainian-controlled territory.

One woman in Luhansk told the BBC on condition of anonymity that she had seen a lot of Russian military armour in the city and the atmosphere now was one of fear and caution.

"I'm scared - it's just scary," she said. Men of military age were required to join the local Russian proxy militia, so anyone who avoided the draft was in hiding, she explained.

"They're mobilising [men] on the streets, catching them. There are no men in the shops, in town, on the streets." As a result, all male-dominated businesses are shut, she said.

"We're already Russia, although just informally. Everyone has a Russian passport."