American officials estimate that nearly 200,000 Russian troops have been killed or wounded in Moscow’s nearly yearlong conflict with Ukraine, The New York Times reported.

The rising number of casualties is driven in part by heavy fighting in Bakhmut and Soledar in eastern Ukraine, where Russia is increasingly sending poorly trained recruits and convicts to the front lines in an effort to deplete Ukrainian artillery, according to the Times.

However, officials have warned that the casualty numbers — which are based on satellite imagery, intercepted communications, social media, on-the-ground reporting and official government numbers — are difficult to estimate and imprecise.

Ukrainian officials have warned recently that Russia has amassed about 500,000 troops on the border in preparation for a new offensive push, as the first anniversary of the war approaches. 

“Now Russia is concentrating its forces. We all know that. It is preparing to try to take revenge, not only against Ukraine, but against a free Europe and the free world,” Ukrainian President Volodymyr Zelensky said on Thursday.

After a series of embarrassing losses last year, Russia has seen some of its first successes in months, gaining ground around Bakhmut in recent weeks.