(NewsNation) —  NATO estimates that 7,000 to 15,000 Russian soldiers have been killed in a month of fighting in Ukraine.

A senior NATO military official said Wednesday that the estimate was based on information from Ukrainian officials, what Russia has released — intentionally or not — and intelligence gathered from open sources. The official spoke on the condition of anonymity under ground rules set by NATO.

The loss of soldiers has led Russia to move reserves from other locations around the world to mobilize in Ukraine.

This war with Ukraine isn’t Russia’s first invasion of a sovereign country. In August 2018, Russian troops invaded Georgia. After a week-long war, Russia took control of two of Georgia’s regions.

Since then, troops have been stationed in the Russian-occupied region; however, recently some troops were moved out and back to Russia so they could be redeployed to Ukraine.

In Chechnya, which is also Russian-occupied, as many as 10,000 troops have been sent to Ukraine to help sustain the Russian army.

Russia has maintained an airbase in Syria since it got involved in that conflict, about 150 Syrian fighters have joined the Ukrainian conflict so far.

Russia also maintains troops in other places like Armenia, Belarus and Moldova, but so far those soldiers have not been moved into the conflict zone.

Ukraine has released little information about its military losses, and the West has not given an estimate. But President Volodymyr Zelenskyy said nearly two weeks ago that about 1,300 Ukrainian servicemen had been killed.

The Associated Press contributed to this report.