Russia's military losses in Ukraine continue to mount. Here's a look at why the death toll is so high

While securing accurate fatality numbers from a war zone is very difficult, evidence is mounting that the Russian military casualty rate in Ukraine is extremely high.

NATO has estimated the number of Russian soldiers killed in Ukraine since the invasion began at between 7,000 and 15,000. That higher estimate roughly equals the number of Soviet soldiers killed in over a decade of fighting in Afghanistan.

According to a report in the New York Times in mid-March, United States intelligence officials said they were confident that up to 7,000 Russians had been killed by that point in the conflict.

The Washington Post reported around the same time that a Russian news website posted a file — and then swiftly took it down again — claiming that up to 10,000 soldiers had been killed so far in the conflict.

CBC News takes a closer look at why Russia's losses have been so high and how long they can be sustained, and the difficulty of getting accurate statistics out of a war zone.

While experts say there are reasons to believe some of the Russian fatality estimates are close to the mark, getting a clear account of the battlefield death toll is almost impossible.

"In war conditions you have the fog of war, which makes it very difficult to get accurate numbers," said Walter Dorn, a professor of defence studies at the Royal Military College.

"In order to see deaths you'd have to go to places where there's people dying, which usually means there is a dangerous threat. So it's hard for objective observers to get that kind of number."

Stephen Saideman, the Paterson Chair in International Affairs at Carleton University and the director of the Canadian Defence and Security Network, told CBC News that experts don't like to place their trust in figures provided by either Russia or Ukraine.

"Each side has an incentive to inflate the damage they do, and deflate the damage that's been done to them," he said. "It's part of every war to do that."

Experts from the U.S. and NATO use models to calculate losses that are informed by intelligence on the ground, satellite imagery and awareness of the Russian military, making them the most trustworthy sources we're likely to get, said Saideman and Dorn.

"We know the size of a Russian battalion, we know how many guys go into a Russian tank, which tank takes four, which tank takes three, and we have plenty of video and pictures," Saideman said. 

Sean Maloney is a professor of military history at the Royal Military College who served as the Canadian army's historian for the conflict in Afghanistan. He told CBC that, based on his knowledge of Russia's military and sources inside of Belarus and Russia, the high-end NATO estimate of Russian casualties is likely accurate.

"I am confident, with the sources that I have, that the number of Russians killed in action is above 15,000," Maloney said.