(CNN) Russia's invasion of Ukraine has been far costlier and less successful than most analysts expected over its first six weeks -- and experts now believe that Moscow is changing its military approach.
President Vladimir Putin's revised war strategy will now focus on trying to take control of the Donbas and other regions in eastern Ukraine with a target date of early May, according to several US officials familiar with the latest US intelligence assessments.
That makes the city of Sloviansk, more than 300 miles east of the capital Kyiv, a potentially crucial battleground in the coming weeks.
"Efforts by Russian forces advancing from Izyum to capture Slovyansk will likely prove to be the next pivotal battle of the war in Ukraine," the Washington DC-based think tank Institute for the Study of War (ISW) said in its Monday update on the conflict in Ukraine. Its report uses alternative transliterations of Ukrainian place names.
ISW expects Russian troops to begin offensive operations towards the city from nearby Izium in the coming days, a forecast that matches warnings on the ground.
"They go south to Kamyanka because it is the road to the city of Sloviansk," Max Strelnyk, a deputy in the Izium city council's office, told CNN late last week of Russian troops' plans. "We have radio interceptions of their talks; their task is to capture the Donetsk region from the north."
On Wednesday, Vadym Denysenko, adviser to Ukraine's Ministry of Interior, said: "If we talk about the key directions where combat will be ongoing -- it's the Sloviansk [Donetsk region] and Barvinkove [Kharkiv region] directions."
Residents of Sloviansk, a city of just over 100,000 people before the invasion, are now being urged to evacuate, while Ukrainian forces are preparing to defend it from a new Russian onslaught.
Control of the city has significant strategic importance in the wider conflict. Should Russian forces overrun Sloviansk, they would be able to cut off Ukraine's forces in the wider region. If they are held back by Ukrainian resistance, however, Russia's ambitions to control both the Luhansk and Donetsk regions will suffer a huge blow.
Sloviansk was a major flashpoint in the war in the Donbas region in 2014, and was briefly held by pro-Russian separatists before they were pushed back by Ukrainian forces in July of that year.
Its significance now lies primarily in the fact that it is surrounded on three sides by Russian-held cities -- Izium to the north, Luhansk to the east, and Donetsk to the south -- but lies deeper west in the Donbas region than the two latter locations, blocking Russia's pathways further into Ukrainian territory.
"Russian forces likely intend to cut off Ukrainian forces in eastern Ukraine and will need to take Slovyansk as their minimum step to do so," the ISW said.
A successful Russian assault on the city would give Moscow the option to link troops up with those fighting in Rubizhne, to the northeast of Sloviansk, or move them south, towards Horlivka and Donetsk, in an attempt to encircle Ukrainian fighters there, the group added.
But Russia appears to have encountered multiple military failures during the first six weeks of the invasion, and its inability to capture cities further west, such as the capital Kyiv, has likely prompted its renewed focus on the Donbas.
In that context, a fresh Russian defeat in the face of Ukrainian resistance could imperil even their new strategy in the east.
"If Russian forces are unable to take Slovyansk at all, Russian frontal assaults in Donbas are unlikely to independently breakthrough Ukrainian defenses and Russia's campaign to capture the entirety of Luhansk and Donetsk oblasts will likely fail," the ISW said.
Reinforced Russian troops currently driving from Izium towards Sloviansk are made up of elements from the 1st Guards Tank Army that had previously been in the Kharkiv-Sumy area to the north, the ISW said on Tuesday.
They do not yet include the units that were withdrawn from the Kyiv region, the institute added. And, it said, the operation toward Sloviansk has "continued on a small scale and made limited progress" so far this week.
But Russia may ramp up its assault on the city when more units become available. The citizens of Sloviansk were given an evacuation order on Monday, amid fears of a full-scale attack and an ensuing humanitarian catastrophe in the city.
Such a fate has already descended on Izium, which has come under sustained Russian attack for weeks. On Monday a senior US defense official told reporters that Russian forces are "still flying about more than 200 sorties a day," and "most of their airstrikes are focusing on the Izium area."
"Every day, it gets worse," Strelnyk told CNN of the humanitarian situation in Izium on Friday.
"There's been no pause in the bombing -- it started weeks ago -- by the Russians. Although Russia claims that they will decrease military operations in the Kyiv and Chernihiv oblasts, Izium and the greater Kharkiv region will have no such luck," Strelnyk said.
"In the city, the dead are buried in the central park of our city," he told CNN. Video footage that has been geolocated and had its authenticity verified by CNN showed dead bodies across the city's central park.
On Wednesday, Ukrainian officials said major fighting was underway in Ukraine's east, with the regional military governor of eastern Luhansk region urging civilians to evacuate some towns. Sloviansk sits in the north of the bordering Donetsk region.
But officials across the two regions have spoken of their difficulties in getting people out or aid in.
Ukraine has repeatedly accused Russia of attacking intended humanitarian corridors and bombing residents of cities as they fled.
Ivan Fedorov, mayor of the Russian-occupied city of Melitopol in the south, said Tuesday that Russian troops had been blocking the delivery of humanitarian supplies and hindering the evacuation of civilians.
"The situation with humanitarian corridors has not improved," Fedorov said in televised remarks. "For the past two weeks, we managed to deliver only two humanitarian cargoes to Melitopol."