The United States has provided intelligence to Ukrainian forces that allowed them to target and kill many Russian generals, according to a report in the New York Times.
Citing unnamed senior officials, the newspaper claims US intelligence have detected the whereabouts of several battlefield headquarters where Russian generals were based.
That information was then shared with Ukrainian units, who used artillery and other targeted strikes to kill the senior Russian officers.
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The US also provided vital information on the location of Russia's Black Sea flagship the Moskva, which was struck and sunk by Ukraine last month.
The guided missile cruiser was sunk after being hit by Ukrainian Neptune missiles after the US identified the vessel, NBC News reported.
The Russian navy has since withdrawn from the Ukraine coast.
Ukrainian forces, having spotted the Russian warship in the Black Sea, called their American contacts for confirmation that it was in fact the Moskva, sources familiar with the events told CNN.
The US responded that it was, and provided intelligence about its location.
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It is not clear whether the US knew Ukraine would move to strike the ship, however, and the US was not involved in that decision, the sources said.
The episode, first reported by NBC News, reflects the Biden administration's increasingly forward-leaning posture when it comes to sharing intelligence with Ukraine, part of a broader policy shift toward helping Ukraine defeat Russia decisively on the battlefield and significantly weaken its military.
But it also raises questions about what the red lines are for the US and Russia when it comes to US military support to Ukraine.
The US has for months provided Ukrainian forces with intelligence about Russian troop movements inside Ukraine, including intercepted communications about Russian military planning.
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It also provides Ukraine with maritime awareness information to allow them to better understand the threat posed by Russian ships in the Black Sea, many of which are firing missiles onto Ukrainian territory.
There are also clear limits, however, to what the US will share, multiple sources told CNN.
The US government declined to confirm they had deliberately disclosed the location of Russian generals.
"We do not provide intelligence on the location of senior military leaders on the battlefield," the Pentagon's press secretary John Kirby said yesterday.
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Generals typically stay far enough back from the frontlines to avoid being hit, with mobile headquarters intended to relocate often enough to prevent detection from the enemy.
Matthew Sussex, a senior fellow at the Centre for Defence Studies at the Australian Defence College in Canberra, told 9news.com.au in March the Russian army could ill afford to lose its senior commanders in combat.
"The deaths of generals as well as other officers such as majors and colonels means the Russian army is losing about 20 years' experience with every one," Associate Professor Sussex said.
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He said junior commanders were ill equipped to take over command because the Russian army has a system of concentrating major decisions with its senior commanders.
While the invasion of Ukraine has been particularly bloody, the deaths of an estimated 12 Russian generals is exceptionally high.
By contrast, only one American general has been killed during wartime since the end of the Vietnam War.
That general, Harold Greene, was killed by a member of the Afghan National Army at a base in Kabul in 2014.
Russia has been concentrating its forces in the east of Ukraine after its attempt to seize Kyiv was a bloody failure.
But their attempted advances in the Donetsk region have been costly and slow-moving.
In Mariupol, a strategically important port city in Ukraine's south, Russia has been unable to claim victory thanks to a tenacious few hundred Ukrainian soldiers holding out in an underground tunnel network underneath the steel plant.
-with CNN
