On the war’s 100th day, the UK said Russia holds 90% of the Luhansk region – half of the Donbas – and is likely to complete control of it in two weeks.
It said these tactical successes had been achieved “at significant resource cost” with the concentration of forces.
But it said none of Russia’s original war objectives had been achieved.
Ukraine’s President Volodymyr Zelensky has warned that Moscow’s forces are now in control of 20% of Ukrainian territory, with a front line that extends for more than 1,000 km (621 miles).
On Thursday, Nato Secretary-General Jens Stoltenberg said Western nations must be prepared to support Ukraine over the long haul, as the conflict becomes a “war of attrition”.
In its most recent count of civilian casualties on 2 June, the UN said it had recorded 4,169 deaths and 4,982 injuries – including 67 deaths of children.
But it said the true total was likely to be considerably higher as it has not yet been able to confirm deaths in some regions where the fighting is intense – such as Mariupol in southern Ukraine, where officials estimate 21,000 civilians have been killed.
Assessing the 100-day conflict so far, the UK’s Defence Intelligence, part of the Ministry of Defence, said Moscow had failed in its initial objectives of seizing Ukraine’s centres of government and the capital, Kyiv.
“Staunch Ukrainian resistance” and the failure to secure Hostomel airfield near the capital during the first 24 hours led to the Russian assault being pushed back, the intelligence update said.
It also said Russian forces suffered from “false planning assumptions” and “poor tactical execution”.
But since it changed its strategy to focus on the Donbas, the region in south-eastern Ukraine where Russian-backed separatist groups have been fighting for eight years, Moscow has begun “achieving tactical success”, the UK said.
“Russian forces have generated and maintained momentum and currently appear to hold the initiative over Ukrainian opposition,” the intelligence update said.
“Russia controls over 90% of Luhansk Oblast and is likely to complete control in the next two weeks.”
The UK intelligence assessment said Russia had not been able to “generate momentum or movement” on other fronts and is on the defensive in other parts of the conflict.
Achieving “any form of success” in the war will require “continued huge investment of manpower and equipment” and is likely to take a considerable amount of time, the assessment concluded.