The United States estimates that Ukraine's drone strike over the weekend hit up to 20 Russian warplanes, destroying about 10 of them, two U.S. officials told Reuters.
The figure they gave is about half the number announced by Ukrainian President Volodymyr Zelensky.
Still, U.S. officials described the attack as extremely significant. One of them warned that it could put Moscow in a much tougher position in US-brokered talks to end the war.
Yesterday, Russian leader Vladimir Putin told US President Donald Trump in a phone call that Moscow would have to respond to the attack.
Trump, in turn, said that it was "a good conversation, but it won't lead to immediate peace".
Ukraine claims to have attacked four air bases in Russia with 117 drones launched from containers near the targets in an operation codenamed "Cobweb".
Yesterday, footage was released showing the drones striking Russian strategic bombers and landing on the dome antennas of two A-50 military spy planes, of which there are only a few in the Russian fleet.
The two US officials, who spoke on condition of anonymity, estimated that the Ukrainian strikes destroyed about 10 and damaged up to 20 military aircraft in total.
However, this estimate is far lower than that presented by Zelensky yesterday. He indicated that half of the 41 Russian aircraft hit were too damaged to be repaired.
Russia, which prioritizes its nuclear forces as a deterrent to the United States and NATO, yesterday called on the United States and Britain to restrain Kiev after the attacks. Russia and the United States together hold about 88% of all nuclear weapons in the world.
The United States says it was not notified by Kiev before the attack.
The war in Ukraine has been escalating despite nearly four months of efforts by Trump, who says he wants peace after Europe's deadliest conflict since World War II.
Ukraine's internal security agency, the SBU, reported that the damage caused to Russia by the operation amounted to $7 billion, and that 34% of the strategic cruise missile carriers at Russia's main airfields were hit.
Commercial satellite images taken after the attack showed that Russian Tu-95 heavy bombers and Tu-22 Backfires - long-range supersonic strategic bombers that Russia has used to launch missile strikes against Ukraine - appeared to be damaged.
The Russian Ministry of Defense The defense admitted that Ukraine attacked airfields in the Murmansk, Irkutsk, Ivanovo, Ryazan and Amur regions, being repelled in the last three locations. The ministry also said several planes had caught fire in the Murmansk and Irkutsk regions. The attack lifted Ukrainian morale after months of relentless Russian pressure on the battlefield and numerous powerful missile and drone strikes. It also showed that, even as it struggles to stop encroaching Russian forces, Kiev can surprise Moscow deep inside its own territory with attacks up to 4,300 km (2,700 miles) beyond the front line. Prominent Russian military bloggers have accused Russian authorities, particularly the Aerospace Command, of negligence in allowing the nuclear-capable bombers to be attacked. Trump's envoy for Ukraine said the risk of an escalation of the war in Ukraine is "significantly increasing."