Ukraine to withdraw from key city of Severodonetsk as Russia's advance grinds on

 Russian soldiers left behind undetonated bombs near Kyiv. See how Ukraine is getting rid of them

(CNN)Ukrainian forces are withdrawing from Severodonetsk, effectively ceding the city to Russia and putting the eastern Ukrainian region of Luhansk almost fully under Russian control following months of grueling and bloody fighting

Severodonetsk was one of the last major Ukrainian strongholds in the area. Serhiy Hayday, a top military commander in east Ukraine, said the military made the decision to evacuate "because the number of dead in unfortified territories may grow every day."

"It makes no sense to stay," Hayday said.

It's unclear if Ukrainian forces are currently leaving the city, or if they have already evacuated.

Though the capture is a symbolic breakthrough for Russia, it comes after a lengthy and costly battle in which Moscow's forces were met with a stubborn Ukrainian resistance.

Russian forces have diverted much of their firepower toward overrunning the city, simply destroying every defensive position the Ukrainians have adopted. The strategy played out slowly, with the Russians making labored and sluggish gains around Severodonetsk throughout the spring and early summer.

Smoke rises over Severodonetsk during heavy fighting between Ukrainian and Russian forces.

Ukrainian forces were, little by little, pushed into a few square blocks around the Azot chemical plant, where some 500 civilians, including dozens of children, have taken shelter -- a scene reminiscent of the siege of the Azovstal steel plant in Mariupol.

With the military evacuating the city, however, the fate of those inside the Azot plant is unclear.

Hayday, the head of the Luhansk regional military administration, has repeatedly accused Moscow of scorched-earth tactics, flattening cities with little regard for casualties as it attempts to take them.

"All the infrastructure of the city is completely destroyed," he said of Severodonetsk on Friday.

The battle now moves across the Siverskyi Donets river to Lysychansk, the last city in Luhansk held by Ukrainian forces. And there are already signs that the Russians will use the same merciless tactic of aerial bombardment to grind down Ukrainian forces, deploying combat planes, multiple launch rocket systems and even short-range ballistic missiles.

Ukraine's control over Lysychansk has become more tenuous in recent days. Russian forces have advanced into several villages south of the city, though not without sustaining losses from Ukrainian artillery fire. The Ukrainian military claims that some Russian battalion tactical groups are being consolidated or withdrawn to restore their combat capabilities.

The Institute for the Study of War, a US think tank that follows the campaign closely, said the Russian breakthrough from the south means they "may be able to threaten Lysychansk in the coming days while avoiding a difficult opposed crossing of the Siverskyi Donets River."

A win for Putin, but at what cost?

Luhansk and neighboring Donetsk together make up Ukraine's Donbas region, an industrial heartland dotted with factories and coal fields that has been home to sporadic fighting since 2014, when Russian-backed separatists seized control of two territories -- the self-declared Donetsk People's Republic and Luhansk People's Republic.

The Kremlin has been quietly supporting troops there since 2014, and even began granting passports to residents in 2018, with more than half a million distributed by mid-2021, according to Russian state media.

Shortly before invading Ukraine in February, Russian President Vladimir Putin recognized the two separatist territories as independent states, ordering the deployment of Russian troops there in defiance of international law.

The capture of Severodonetsk gives Putin an important propaganda victory in a war which, so far, has been mostly marked by Moscow's military setbacks. A key goal of Russia's so-called "special military operation" -- the Kremlin's official euphemism for the invasion of Ukraine -- was to take control of Donbas.

Experts expected a quick fight in the region, unlike the battles around Kyiv in the initial days of the war that Russia lost. The fighting near the Ukrainian capital was mostly urban warfare, which allowed Ukraine's military to stymie Russia's advantages in manpower and hardware by keeping the battles in tighter corridors, where Ukraine's highly motivated fighting force could capitalize on its better knowledge of the local environment.

Donbas, however, is a region of plains and open spaces. The battles there have involved long-range weaponry, a type of warfare that favors Russia and its superior power and larger armed forces.

After little success in the first month of the conflict, Russian forces retreated from around Kyiv, regrouped and concentrated on eastern Ukraine. The Kremlin's new offensive to take the Donbas region was launched April 18, according to Ukraine's President Volodymyr Zelensky. Russia's progress was, initially, "slow and uneven," according to US officials, as its army appeared to still be learning from its mistakes in the initial days of the invasion.

A boy sits on the rubble of a building hit in a strike on Kramatorsk, a city in the Donetsk region.

The tide began turning in mid-May, when Mariupol, a strategically important port city, finally completely fell to Russian forces following an intense, three-month long bombing campaign that Ukrainian officials say left as many as 22,000 people dead. The fight there was markedly similar to the battle for Severdonetsk, both in terms of Russia's tactical decisions and with Ukrainian fighters and civilians holing up in structures that, before the war, were used for heavy industry.

Russian then increased the intensity of their bombardment in other parts of the Donbas region, a strategy Zelensky likened to genocide.

Eyes shift to Donetsk

Some experts have questioned whether Russia's efforts to take Severodonetsk were strategically worth it.

"The loss of Severodonetsk is a loss for Ukraine in the sense that any terrain captured by Russian forces is a loss -- but the battle of Severodonetsk will not be a decisive Russian victory," said the Institute for War.

"Ukrainian troops have succeeded for weeks in drawing substantial quantities of Russian personnel, weapons, and equipment into the area and have likely degraded Russian forces' overall capabilities while preventing Russian forces from focusing on more advantageous axes of advance."

If Russian forces capture Lysychansk, and with it the Luhansk region, they will likely concentrate more troops on Donetsk, where progress has come much more slowly.

Ukraine's regional military administration says about 45% of Donetsk is held by Ukrainian forces, including the cities of Sloviansk and Kramatorsk.

It's unclear whether the losses inflicted on Russian forces in recent weeks will impair their ability and desire to gobble up more territory, but the Kremlin has not veered from its ultimate objective of taking those two cities.

Equally, it remains to be seen whether the punishment endured by Ukrainian units has left them with enough resources to launch counterattacks against the Russians.

Ukrainian officials have made repeated calls for more military assistance from its allies. Deputy Defense Minister Hanna Maliar said on June 14 that the country had received just 10% of military assistance it had requested.

"No matter how professional our army is, without the assistance of our Western partners Ukraine will not be able to win this war," said Maliar.

Ukrainian commanders will now have to decide whether it is strategically worthwhile to keep defending Lysychansk, as Kyiv could abandon the city and divert resources for a more consolidated defense of Sloviansk, Kramatorsk and Kostiantynivka, the industrial belt of Donetsk.

The Kremlin has not veered from its ultimate objective of taking all of Donetsk and Luhansk. It now has almost all of the latter. But completing the so-called "special military operation" will likely take many more months, setting up a war of attrition.

CNN's Nathan Hodge, Julia Presniakova, Olga Voitovych, Oleksandra Ochman, Rebecca Wright and Rob Picheta contributed to this report.