Roughly 1,000 Russian mercenaries, known as the Wagner Group, with combat experience in Libya and Syria, are concentrating in Ukraine's Donbass region.

Despite the buildup, according to The Washinton Post, the Russian reinforcements are unlikely to change the course of the war.

"The Wagner Group is a private military contractor for Russia," Pentagon spokesman John Kirby said, according to The New York Times. "We know that they have interest in increasing their footprint in Ukraine."

Jim Townsend, the deputy assistant secretary of defense for Europe and NATO during the Obama administration, while not referring directly to the Wagner Group, said, "if [Russia] are substituting the contract soldiers for the conscripts, who have been drafted and aren't happy and aren't as well trained, then that could be helpful. They're probably better trained as well, and their morale will certainly be higher than the conscripts that came in thinking they were going to liberate the Donbass and instead found themselves mired around Kyiv."

While word of Russia's vast arsenal of chemical, biological and tactical nuclear weapons has fueled speculation about what Russian President Vladimir Putin may do if he feels he is running out of options in Ukraine, the fact that he appears to be moving more troops into Ukraine may be a sign that he plans to hold his most desperate moves in reserve.

"I think tactical nuclear weapons are actually somewhat unlikely because of this. It's hard to use them in a way that doesn't blow back onto their own people," Dmitry Gorenburg, a Russian military expert at a Washington think tank, said.

Gorenburg adds that Putin bringing in more mercenaries or troops may not be of much use in advancing his position in Ukraine.

"I'm not sure that anything that the Russian military will do will change the balance; it's more about patching up gaps," Gorenburg said. "They're going to be brought in as fresh bodies, primarily, to replenish the losses. … Folks who are suggesting they're bringing in everybody they can for one big push to break Ukrainian resistance, I don't see that as possible now."

On Friday, Russian military forces signaled that they would be shifting focus to the Donbass region. However, it is unclear if this is a tactic to distract from some other offensive.

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