Russian volunteers are not willing to go into combat.
The official said that Russian forces are not doing well in Ukraine.
Moscow has been hampered by personnel issues.
The US official said that Russia is having a hard time finding volunteers to fight in Ukraine because of devastating losses and poor battlefield performance.
A senior US defense official told reporters on Monday that a personnel shortage has arisen because of the damage done to Russian troops by the counteroffensive by the Ukrainians.
The official said that the news from Kharkiv province inspired many Russian volunteers to refuse combat.
The official said that the Wagner Group is having recruitment problems of its own.
Over 1,500 convicted felons have been offered the chance to fight the Ukrainians if they take up arms but die if they don't. Many of the prisoners are rejecting the offer.
The official said that the information they have indicates that Wagner has been suffering a lot of losses.
According to a recent intelligence assessment by Britain's defense ministry, the push to recruit prisoners and shorten training courses at Russian military academies indicates critical shortages for Moscow.
The impact of Russia's manpower challenge has become more severe.
Russian forces have suffered a lot of casualties as Ukrainian troops advance along multiple fronts.
Russian troops have been hampered by exhaustion and other problems. The manpower issue is compounded by the fact that Putin calls the war a special military operation instead of what it actually is.
The Russian troops who left behind gruesome scenes in their retreat left behind significant advances to liberate territory occupied for months by them.
In an attempt to hold on to captured territory, pro-Moscow rebels in the occupied eastern Ukraine's Donbas region announced on Tuesday that they would hold referendums on joining Russia, a move that both Ukraine and the West have decried in the past as a sham.
Business Insider has an article on it.