Russia refocused its military operation in Ukraine to the Donbas region weeks ago, but it has so far had limited success, according to the Pentagon.

A senior U.S. defense official told reporters on Monday that Russia's military in the Donbas has operated in a "very cautious, very tepid, [and] very uneven" manner, making progress that is "minimal at best."

The Donbas region is in the eastern part of Ukraine, and there is a significant pro-Russian separatist presence where fighting between the two sides has been occurring since 2014. Russia is hoping to gain control of the Donbas region and the land territory that connects it to the Crimean Peninsula, which Moscow annexed in 2014.

“In some cases, quite frankly, the best word to describe [the Russians’ Donbas phase] would be anemic,” the official explained, adding that many of the logistical problems that slowed down their initial phase of the war remain.

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Russian forces are still “suffering from poor command control, low morale in many units,” as well as a "casualty aversion," the official said.

Gen. Valery Gerasimov, the chief of the general staff of the Russian military, traveled to the Donbas last week, and there are reports that he narrowly escaped an attack that killed some 200 Russian troops, though the Pentagon couldn't corroborate the latter.

The Russians committed roughly 120 battalion tactical groups to its military operation and the United Kingdom's Defense Ministry said on Monday that it’s “likely” that “more than a quarter of these units have now been rendered combat ineffective.”

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The Ukrainian Ministry of Defense reported on Monday that Russia has lost more than 23,000 troops, more than 1,000 tanks, and around 350 aircraft and helicopters, while the Russians have been much less open about its losses. Death estimates in war are difficult to track accurately, and both sides in a conflict have motivations to alter the numbers in one way or another.