A U.S. assessment concluded that Ukraine's weekend drone strike damaged up to 20 Russian warplanes and destroyed about 10, U.S. officials told Reuters, revising down Ukrainian President Volodymyr Zelenskyy's higher estimate.
Still, the U.S. officials described the attack as highly significant, with one of them cautioning that it could drive Moscow to a far more severe negotiating position in the U.S.-brokered talks to end more than three years of war.
Russian President Vladimir Putin told U.S. President Donald Trump in a telephone conversation on Wednesday that Moscow would have to respond to the attack, Trump said in a social media post.
Trump added it "was a good conversation, but not a conversation that will lead to immediate peace."
Ukraine says it targeted four air bases across Russia using 117 unmanned aerial vehicles launched from containers close to the targets, in an operation codenamed "Spider's Web."
It released footage Wednesday showing its drones striking Russian strategic bombers and landing on the dome antennas of two A-50 military spy planes, of which there are only a handful in Russia's fleet.
The two U.S. officials, who spoke on the condition of anonymity, estimated the Ukrainian strikes destroyed around 10 and hit up to 20 warplanes in total.
That estimate is far lower than the one Zelenskyy offered to reporters in Kyiv earlier Wednesday. He said half of the 41 Russian aircraft struck were too damaged to be repaired.
Reuters could not independently verify the numbers from Kyiv or the United States.
Russia, which prioritizes its nuclear forces as a deterrent to the United States and NATO, urged the United States and Britain on Wednesday to restrain Kyiv after the attacks. Russia and the United States together hold about 88% of all nuclear weapons.
The United States says it was not given any notice by Kyiv ahead of the attack.
The war in Ukraine is intensifying despite nearly four months of efforts by Trump, who says he wants peace after the deadliest conflict in Europe since World War II.
The White House did not immediately respond to a request for comment.
The Russian and Ukrainian embassies also did not immediately reply.
Ukraine's domestic security agency, the SBU, said the damage to Russia caused by the operation amounted to $7 billion and 34% of the strategic cruise missile carriers at Russia's main airfields were hit.
Commercial satellite imagery taken after the Ukrainian drone attack shows what experts told Reuters appear to be damaged Russian Tu-95 heavy bombers and Tu-22 Backfires, long-range, supersonic strategic bombers that Russia has used to launch missile strikes against Ukraine.
Russia's Defense Ministry has acknowledged that Ukraine targeted airfields in the Murmansk, Irkutsk, Ivanovo, Ryazan and Amur regions and were repelled in the last three locations. It has also been reported that several aircraft caught fire in the Murmansk and Irkutsk regions.
The attack has bolstered Ukrainian morale after months of unrelenting Russian battlefield pressure and numerous powerful missile and drone strikes by Moscow's forces.
It also demonstrated that Kyiv, even as it struggles to halt invading Russian forces, can surprise Moscow deep inside its own territory with attacks up to 4,300 kilometers (2,670 miles) from the front lines.
Influential Russian military bloggers have accused Russian authorities, especially the aerospace command, of negligence and complacency for allowing the nuclear-capable bombers to be targeted.
Trump's Ukraine envoy said the risk of escalation from the war in Ukraine was "going way up," particularly since Kyiv had struck one leg of Russia's "nuclear triad," or weapons on land, in the air and at sea.
"In the national security space, when you attack an opponent's part of their national survival system, which is their triad, the nuclear triad, that means your risk level goes up because you don't know what the other side is going to do," Trump's envoy, Keith Kellogg, told Fox News on Tuesday.
