Russia has probably lost around a third of the ground forces it deployed to Ukraine and its offensive in the Donbas region "has lost momentum and fallen significantly behind schedule", British military intelligence said on Sunday.
"Despite small-scale initial advances, Russia has failed to achieve substantial territorial gains over the past month whilst sustaining consistently high levels of attrition," the British defence ministry said on Twitter.
"Russia has now likely suffered losses of one third of the ground combat force it committed in February."
It said Russia was unlikely to dramatically accelerate its rate of advance over the next 30 days, reports Reuters.
Since Russia's invasion on Feb 24, Ukraine's military has forced Russia's commanders to abandon an advance on the capital Kyiv, before making rapid gains in the northeast and driving them away from the second biggest city of Kharkiv.
A Ukrainian counteroffensive has been underway near the Russian-held town of Izium, though Ukraine's military reported on Sunday that Russian forces were advancing elsewhere in the Donbas region, the main theatre of war over the past month.