Russia is struggling to make progress in the Donbas and is likely to come across more heavily fortified Ukrainian positions as its armed forces look to advance, Western officials have said.
Moscow's invading troops are using "reckless" manoeuvres - as seen during a failed river crossing which is said to have resulted in heavy losses - in a bid to "take the fight to the Ukrainians" in the east of the country, they said.
But even small gains are being quickly taken back by Kyiv counter attacks amid fears in the West that the battle for the region will settle into a long and attritional conflict.
One official said: "Those gains that are being made are one or two kilometres at a time.
"And then the Ukrainians have been very effective at launching counter offensives almost immediately, so again we see the front line sort of oscillate.
"Those gains are small, they remain focused largely on villages and towns, achievable goals. `
`And they will be facing harder challenges in the future.
"For example, Rubizhne has now fallen. It took the Russians two-to-three weeks to achieve that objective. That's much longer than they would have anticipated.
"As they attempt to move to Kramatorsk, that's going to be a much more heavily defended and fortified urban environment for them to tackle. So the advances the Russians have made are somewhat limited."
Reports had suggested that Russian president Vladimir Putin hoped for a quick march on the Donbas in time to mark Victory Day on May 9, but that alleged target has come and gone with the regional battle still raging.
Despite withdrawing from an offensive on capital Kyiv and refocusing on the east, the struggle for the Donbas has turned into a grinding, village-by-village assault.
It has also come with Russian losses, with the Ukraine's airborne command releasing photos and video this week of what it said was a damaged Russian pontoon bridge over the Siversky Donets River and several destroyed or damaged Russian military vehicles nearby.
The UK's Ministry of Defence (MoD) said Russia lost "significant armoured manoeuvre elements" of at least one battalion tactical group during the operation.
One official said: "Trying to do river crossings in the middle of the day is what some might call reckless.
"But they still pose a very real threat to Severodonetsk and trying to encircle the Ukrainian forces in that area. They will continue to make small gains towards that objective."
Russian fighters are being prevented from joining the Donbas push as they struggle to flush out the last remaining Ukrainian defenders from Mariupol, western officials said.
The occupation of the warren of tunnels underneath the Azovstal steel plant has meant some of the Kremlin's troops, including special forces and artillery, has been "tied down" in the battered coastal city.
"They are not there in large numbers but it is absolutely a thorn in the side of the Russians and it will prevent them, because the steel work is contiguous with the port area, from using the port with any level of assurance," the official said.
Ukraine needs more advanced weapons from the West, however, if it is to retake large swathes of conquered ground.
One official said Kyiv was "clearly" capable of making advances by applying "intense pressure" on Russian invaders, but was struggling to get the upper hand over its rival.
The Ukrainians are dug into defensive positions in the Donbas, having been fighting in the region against Russian-backed separatists since 2014, making large offensives on their invaders more difficult, the officials said.
But using the tactic of deploying smaller units had allowed the Ukrainians to become more agile in pushing Moscow's troops back.
"The reason the Ukrainians are asking for more Western support and weapons and more advanced weapons is to help overcome the capability overmatch they face from the Russians, which would then allow them to conduct operations which allow them to take entire oblasts," an official said.