Putin is preparing an eastern offensive in Ukraine. Here's how this phase of the war could unfold

Russian President Vladimir Putin is overseeing a change in military strategy to the Kremlin's unprovoked onslaught in Ukraine, repositioning forces in the east of the country in a bid to take control of the Donbas region.

Analysts see the shift in Russia's approach as a tacit acknowledgment of failure, saying fierce resistance from Ukrainian forces has thwarted Putin's attempt to swiftly capture major cities and topple the government.

The next phase of the war is likely to result in a dangerous stalemate, according to analysts, exacerbating an already devastating humanitarian crisis as Russia's top army commanders seek to establish full control over the breakaway republics of Donetsk and Luhansk.

Ukraine's deputy prime minister on Wednesday urged people in the eastern regions of Kharkiv, Donetsk and Luhansk to evacuate amid growing fears of an imminent attack. "It is necessary now, because then people will be under fire and threatened with death," Iryna Vereshchuk said.

It comes less than two weeks after Sergei Rudskoy, deputy chief of staff of Russia's Armed Forces, announced that troops were shifting away from a countrywide attack. Instead, Rudskoy said the Kremlin's goal was to concentrate efforts on the "complete liberation" of the Donbas region.

"It seems to me that this is the biggest single piece of news since the war began," Christopher Granville, managing director of EMEA and global political research at TS Lombard, told CNBC via telephone. "I thought it at the time, and I haven't changed my mind ... since then."

Granville said Russia's reconcentration of troops in the east of Ukraine foreshadows "some all too plausible horrors." He cited particular concern for Slovyansk and Kramatorsk, two sizable cities situated in the north of the Donetsk Oblast.

Thousands of people have been trying to flee the Donbas region, with scores of families seen queuing for days at Kramatorsk's central rail station in an attempt to reach safety.

For some, the situation is all too familiar.

Ukrainian forces fought Russian-backed separatists in Kramatorsk in 2014, and Granville said the nearby city of Slovyansk was known to have "totemic significance" for the Donbas separatists.

Russia has not yet committed forces withdrawn from the so-called Battle of Kyiv to an eastern offensive, according to analysts at the Institute for the Study of War, but troops are thought to be preparing for an attack on Slovyansk.

"I think from a military point of view, there has to be a question about success and morale. Russia, having soldiers sitting around Kyiv being shot at, what is the purpose? What are they trying to do?" Granville said.

"It's just common sense that soldiering needs to have a goal, and the natural goal for soldering is to get territory. This is the campaign in the Donbas," he said. "The soldiers who are fighting can see what they are fighting for, they can see progress. And I think that goes from the senior levels of Russian general staff to the commanders and men in the field."

Jonathan Flint, a military strategist and adjunct professor at Case Western Reserve University in Cleveland believes there are two pathways for Russia to try to achieve its new military aims.

"One would be for Russia to withdraw to relative safety, and use this opportunity to rearm, reorganize and reinforce their forces for a better organized and more competent advance back into Ukrainian-held territory," Flint told CNBC.

This approach is not without risks, however, particularly given that Ukrainian forces could cross borders to engage with Russia and a second attempt to invade may fail as it did in the first instance, he said.

"The other option would be to entrench in these areas, making it all but impossible for them to be retaken by Ukrainian forces and returned to Ukrainian control," Flint said. "This may ultimately prove the wiser route for Russia, because by solidifying a frozen conflict it would essentially stop Ukraine from joining the EU or NATO in the future despite any undertakings made not to during peace negotiations."

Bruno Lete, senior fellow of security and defense at The German Marshall Fund, told CNBC that while Russian forces had lost the battle for Kyiv, the Kremlin's nearly 6-week-old war is not over yet.

"Beyond the east, we must also look at the south of Ukraine. Already vast areas of Ukraine's coastlines east of Crimea are occupied," Lete said. "Clearly Russia is trying to establish a land bridge between Crimea and Russia. If Mariupol falls, Russia will have succeeded."

Heavy fighting and Russian airstrikes continue in Mariupol, British military intelligence reported Wednesday, in a move likely designed to pressure Ukrainian forces in the encircled southeastern city to surrender.

The U.K. Defense Ministry estimated that most of the remaining 160,000 residents of Mariupol have no access to electricity, communication, medicine, heat or water — underscoring the worsening humanitarian crisis there.

Lete said Russia may also consider intensifying attacks on the strategically important port hub of Odesa on the Black Sea coast to establish a coastline bridge from Crimea to Transnistria — a Moldovan breakaway region occupied by Russian forces.

"Ukrainians have the ability to defend themselves on land, but far less so in the air. … Therefore the first stage of these next battles will be characterized by Russia conducting missile attacks and airstrikes on critical and civilian infrastructure," he added.

Russia's retreat from the suburbs of Kyiv has coincided with an outpouring of international condemnation as world leaders reacted in horror to the mounting evidence of war crimes.

The Kremlin has denied allegations of executing civilians and, without evidence, accused Ukraine of navigating a cynical ploy to denigrate the Russian army.

Ukrainian President Volodymyr Zelenskyy has accused Russia of committing genocide in Ukraine, while U.S. President Joe Biden has called for Putin to be tried for war crimes.

Fabrice Pothier, CEO of political consultancy Rasmussen Global, said Russia's objective seemed to be to consolidate the territorial hold that the Kremlin had in eastern Donbas since 2014.

"I think this is a game of who can hold longer and who can convince, basically, the civilian population that the fight is worth the cost," Pothier told CNBC's "Squawk Box Europe" on Tuesday. "I think for the moment Zelenskyy is on strong ground as long as he gets the right kind of support from the West."

Putin, on the other hand, is seen to have strong support from within Russia, Pothier said, but for how long is uncertain. "I think there is going to be a moment of truth, [a moment] of reckoning for the Russian leader vis-a-vis his population."

Ultimately, TS Lombard's Granville said Russia's offensive was likely to become a war of attrition. "It seems to me that the Russian stance will become more defensive … and this is a formula for a very protracted conflict."

Flint, too, was skeptical of an imminent breakthrough in peace talks. "Only when one side feels the pain is intolerable do I expect to see movement towards peace," he said.