According to Western officials, more than 40,000 of Vladimir Putin’s troops have been wounded in action in Ukraine since the conflict began more than three months ago.

They estimate that over 15,000 Russian military personnel have been killed, though “not necessarily significantly more” than the figure given several weeks ago, as President Putin has scaled back his military campaign to focus on the Donbas region in the country’s east.

“The number of Russian wounded in action is probably north of 40,000,” said one official.

This high figure is thought to be due in part to how Russian generals have led troops into perilous advances, the strength of Ukrainian resistance, and weapons supplied by Western nations. However, Western officials believe Mr Putin’s forces were deployed with insufficient medical support, partly because he mistook them for liberators.

“Some units deployed in this operation have deployed without anything remotely resembling adequate medical support, so when they do take casualties, those casualties cannot be triaged and treated appropriately,” one official explained.

Ukrainian casualties are estimated to range between 60 and 100 soldiers per day.

According to western officials, Mr Putin’s troops are advancing at as little as 500 meters per day in parts of eastern Ukraine as they face fierce resistance.

They emphasized that Russian generals would have planned to gain territory at tens of miles a day.

Mr Putin’s military operation is focused on capturing territory in the Donbas region, and his forces are thought to have captured more than half of Severodonetsk.

Heavy artillery bombardments are leading the Russian offensive, razing towns, cities, and villages in south-east Ukraine before troops attempt to advance.

“It’s making slow but steady gains, largely through its doctrine of concentrating indirect fire, immense firepower, and then grinding through the movement,” one official said. “It’s worth looking at the axis from Popasna, where forces have been advancing against that pocket.

“In terms of progress, they have averaged between 500 meters and a kilometer (0.62 miles) per day over the last month.” Doctrinally, an advance of this magnitude would require tens of kilometers.

According to two senior US administration officials, they are part of a new $700 million (£553 million) tranche of security assistance for Ukraine that will include helicopters, Javelin anti-tank weapon systems, tactical vehicles, spare parts, and more.

While the Kremlin’s military campaign is making progress, it is also vulnerable on its operational flanks, with Ukrainian forces launching limited counter-attacks and seizing some territory in the Kherson province that Russia “relatively lightly holds.”

Western officials believe that such a scenario will play out in the coming months, with Mr Putin’s troops being targeted in areas where they do not have a large presence.

His generals, on the other hand, appear to have learned from some of the blunders that forced them to abandon his lightning invasion plan, which included seizing Kyiv in days, and to retreat from the capital and swaths of northern Ukraine.

Given the risk of air defense systems, the Russian air force is still believed to be largely operating over its own positions rather than Ukrainian ones, limiting its impact “in depth.”

If Mr Putin’s forces capture Severodonetsk, they will face more difficult objectives, such as seizing the key railway city of Kramatorsk in order to gain control of the entire Donbas.

The Russian president is still believed to be intent on capturing large swaths of Ukraine beyond the Donbas, despite the fact that they are “a very long way from Russian ability to deliver on them at the moment.”

Given the massive loss of troops and equipment, Russian ground forces are believed to have been “written down” to just over 50% combat effectiveness, and they have had to resort to rolling out 50-year-old T-62 tanks.