Ukrainian forces have advanced up to five miles and liberated at least four villages since launching their counter-offensive against Vladimir Putin's army, western defence chiefs confirmed on Wednesday.
However they warned that the advance has been slow and the Russian's "well prepared" defences have proved "costly" to Kyiv's troops.
“Ukrainian forces have liberated at least four villages and advanced at least seven kilometres," western officials said in a briefing.
“Russian forces have generally put up a good defence from their well prepared defended positions and have been falling back between tactical lines.
“This manoeuvre defence approach is proving challenging for the Ukrainians and also costly to attacking forces.
“Hence, the advance at the moment has been slow.”
Putin has claimed that Ukraine's counter-offensive, launched on June 4, has been unsuccessful, with its army suffering major losses.
Speaking at a meeting of war correspondents, the Russian President said Kyiv's losses were approaching "catastrophic" levels.
Ukrainian President Zelensky has denied his counter-offensive is failing.
Western officials said this is more than “probing operation” and progress was “slow” as Ukrainian forces through minefields.
“These are early days in the Ukrainian counter-offensive,” they added. “Ukraine maintains committed, quality forces [but] the conflict is likely to be characterised by grinding costly warfare for many months to come.”
Meanwhile, Ukrainian generals have still to send in the “bulk” of their counter-offensive troops, and future battles to recapture land are likely to be far more ferocious, British military experts warned on Wednesday.
In an update on the conflict, Dr Jack Watling, from the London-based Royal United Services Institute, said: “Ukrainian troops have broken through initial fighting positions along a broad part of the front, but remain some distance from Russia’s main defence line.
“Kyiv has yet to commit the bulk of its forces as its lead elements try to set the conditions for a breakthrough.”
He stressed that Russia’s initial fighting positions were made up largely of “fox holes” and hand-dug trenches, but behind these were complex minefields of anti-tank and anti-personnel mines, covered by Russian drones and artillery.
The main Russian defence line was still 15–20 km (nine to 12 miles) from Ukrainian positions, with “properly dug trenches and concrete-reinforced firing posts, tank obstacles, ground-laid cable to coordinate artillery strikes, and even more mines”.
Behind these defences are reserve fighting positions of the third defence line.
“The fighting will likely get tougher,” added Dr Watling, a senior research fellow in land warfare and military sciences at the RUSI think tank.
“As Ukrainian forces penetrate deeper into the defences, they will come into range of more Russian artillery firing posts.
“Moreover, their own artillery will be able to deliver fewer counterbattery missions, and the Ukrainian lines of advance will become more predictable, as they must follow the breaches identified in the minefields.”
Ukraine’s president Volodymyr Zelensky said on Tuesday that “battles are fierce, but we are moving forward” after his military chiefs reported seven villages had been recaptured in the summer counter-offensive so far.
Fighting has been taken place in a series of locations on the frontline in the eastern Donetsk province and more southern Zaporizhzhia province.
But amid a highly complex picture, Russian forces were believed to be launching counter attacks after losing territory to Ukrainian soldiers.
The Ukrainian army is thought to have suffered losses of equipment, with significant casualties in some areas, as it seeks to advance.
Attacking operations normally see higher casualty rates than defensive ones.
