Heavy losses leave Russia short of its goal, U.S. officials say.

WASHINGTON — The staggeringly high rate of Russian casualties in Ukraine means that President Vladimir V. Putin may not be able to achieve one of his key war objectives: seizing the entire eastern region of the country this year, officials in the Biden administration and military experts say.

With 500 Russian troops killed or wounded every day, according to the latest estimate by American intelligence and military officials, Russia’s war effort has decelerated to a grinding slog, the officials said.

Russia’s glacial pace in the east has been further stymied by the arrival of American multiple-launch rocket systems, which have allowed Ukrainian troops to take back some territory and made it more difficult for Russian soldiers to reach other areas.

Earlier this summer, Russian forces captured the Luhansk region of Ukraine, the easternmost part of the country. But in neighboring Donetsk, their progress has stalled, in no small part because of heavy casualties, American military officials said.

“I think it’s safe to suggest that the Russians have probably taken 70 or 80,000 casualties in less than six months,” Colin Kahl, the under secretary of defense for policy, told reporters at the Pentagon on Monday, referring to deaths and injuries.

“They have made some incremental gains in the east, although not very much in the last couple of weeks, but that has come at extraordinary cost to the Russian military because of how well the Ukrainian military has performed and all the assistance the Ukrainian military has gotten.”

Two American officials said that estimate of Russia’s losses included about 20,000 deaths. Of that number, 5,000 are believed to be mercenaries from the Wagner Group, a private force with ties to Mr. Putin, and foreign fighters, one of the officials said, speaking on the condition of anonymity because she was not authorized to discuss sensitive military assessments.

American officials say their casualty estimates are based on satellite imagery, communication intercepts, social media and on-the-ground media reports.