Between 20,000 and 30,000 Russian troops have been reportedly killed and wounded in the battle for the Ukrainian city of Bakhmut since it began last summer.
According to the Western officials, since the fighting began, some 90% of its pre-invasion population has fled.
For Ukraine, one official said, the battle for Bakhmut has been “a unique opportunity to kill a lot of Russians”.
Ukraine’s military has also paid a heavy price, although Western officials reject figures cited on Tuesday by Russia’s defence minister.
In a statement to a defence meeting, posted on Telegram, Sergei Shoigu said Ukraine had lost 11,000 troops in February alone.
Western officials say they “don’t recognise” Mr Shoigu’s figures.
This isn’t the first time Kyiv’s Western backers have suggested that Moscow’s campaign has come unstuck. Similar claims were made following Ukraine’s lightning counter-offensives last autumn.
But Russia’s territorial gains over the winter have been extremely modest.
Meanwhile, Ukraine has been receiving fresh batches of Western military equipment, including tanks and other armoured vehicles, and laying plans for its own offensive, which could come as early as May.