Washington has conveyed its concerns to Kyiv about two attempted attacks over the last week against radar stations that provide conventional air defense as well as warning of nuclear launches by the West. At least one strike in Armavir, in Russia’s southeastern Krasnodar region, appeared to have caused some damage.

“These sites have not been involved in supporting Russia’s war against Ukraine,” the U.S. official said. “But they are sensitive locations because Russia could perceive that its strategic deterrent capabilities are being targeted, which could undermine Russia’s ability to maintain nuclear deterrence against the United States.”

A Ukrainian official familiar with the matter, however, said that Russia has used the radar sites to monitor the Ukrainian military’s activities, particularly Kyiv’s use of aerial weaponry, such as drones and missiles. The official, speaking on the condition of anonymity to discuss a sensitive security matter, confirmed that Ukraine’s Military Intelligence Directorate, known by its initials as GUR, was responsible for the strikes.

Ukraine is facing a continuing threat to its existence from a Russian enemy force — which boasts the world’s largest nuclear arsenal — that has gained ground of late, in part due to its sophisticated radar and weapons-jamming technology, which has rendered virtually useless some U.S.-provided guided missiles and artillery shells. This capability has also enhanced Moscow’s ability to track British and U.S.-provided longer-range weaponry and drones, which have caused serious damage to Russia’s Black Sea fleet and military installations in Crimea, the southern peninsula illegally seized from Ukraine in 2014.

The Ukrainian official said the goal of the strikes was to diminish Russia’s ability to track the Ukrainian military’s activities in southern Ukraine. The drone that targeted the radar station near Orsk, in Russia’s Orenburg region along Kazakhstan’s northern border, traveled more than 1,100 miles, making it one of the deepest attempted strikes into Russian territory. The Ukrainian official declined to say whether the strike, on May 26, caused any damage.

U.S. officials said they are sympathetic to Ukraine’s plight — administration officials are actively weighing whether to lift restraints on the use of U.S.-provided weapons to strike inside Russia. But were Russia’s early-warning capabilities to be blinded by Ukrainian attacks, even in part, that could hurt strategic stability between Washington and Moscow, the U.S. official said.

“Russia could think it has a diminished ability to detect early nuclear activity against it, which then could become an issue,” the official said. “It should be obvious to everyone that there’s no intention whatsoever [by the United States] of using nuclear weapons against Russia. But there’s certainly concern about how Russia could perceive its deterrent capabilities being targeted and early-warning systems being attacked.”

The perception issue is likely fueled by “an erroneous conviction that Ukraine’s targeting is directed by Washington,” said Dmitri Alperovitch, security analyst and chairman of Silverado think tank. “But that means attacks by Kyiv on Russian nuclear deterrence infrastructure has potential to trigger a perilous escalation with the West. At the end of the day, nuclear command and control and early-warning sites should be off-limits.”

Some analysts were puzzled at the targets: While Krasnodar is close enough to Ukraine to track missiles and drones, the radar station near Orsk is focused on the Middle East and China, they said.

Asked why they would target a site so far away, the Ukrainian official asserted that Russia “switched all of its capabilities for war against Ukraine.”

Following Ukraine’s disappointing counteroffensive last year, Russia has regained the initiative on the battlefield in recent months, advancing in the eastern Donetsk region and recently launching a new assault in the northeastern Kharkiv region along the border. Kyiv, meanwhile, has with increasing frequency targeted sites deep in Russia — a capability many doubted was possible without Western support and sign-off.

About three weeks ago, shortly after Russia began its assault on Kharkiv, Ukraine asked the United States to ease long-standing restrictions on using U.S.-provided weapons to attack targets inside Russia. Some senior officials favor such a move, including Secretary of State Antony Blinken, who has urged President Biden to agree to lift the restraints. The White House is considering such a proposal, but no action has been taken yet, officials say.

At a news conference Wednesday in Moldova, Blinken said the United States has “not encouraged or enabled strikes outside of Ukraine, but Ukraine, as I’ve said before, has to make its own decisions about the best way to effectively defend itself.”

Blinken added that the United States has “adapted and adjusted” to changing conditions on the battlefield and that as Russia pursues new tactics of “aggression” and “escalation,” was “confident that we’ll continue to do that.”

There is no restriction on Ukraine using U.S.-supplied air defenses to shoot down Russian missiles or fighter jets over Russian territory “if they pose a threat to Ukraine,” the U.S. official said.

But U.S. officials have previously expressed concern to Ukrainian officials over Kyiv’s attacks on Russian soil, sometimes even intervening during the planning stage. Ahead of the one-year mark of the war, the GUR was planning attacks on Moscow, according to a leaked classified report from the U.S. National Security Agency that was later confirmed by two senior Ukrainian military officials.

Days before the attack, U.S. officials asked Kyiv to scrub their plans, fearing it could provoke an aggressive response from the Kremlin; the Ukrainians complied, according to the leaked U.S. documents and the senior Ukrainian officials.

In a more recent example, Washington took exception to Ukrainian drones targeting oil refineries inside Russia — a request that came directly from Vice President Harris to Ukrainian President Volodymyr Zelensky at the Munich Security Conference in February, according to officials familiar with the matter. U.S. officials believed the strikes would raise global energy prices and invite more aggressive Russian retaliation inside Ukraine.

Amid growing concern over Russia’s battlefield advances, Washington is facing pressure from NATO and several key European allies to allow Ukraine to use the full force and range of U.S.-provided weapons.

“If you cannot attack the Russian forces on the other side of the front line because they are on the other side of the border, then of course you really reduce the ability of the Ukrainian forces to defend themselves,” NATO Secretary General Jens Stoltenberg, the alliance’s top political official, said during a visit to Bulgaria on Monday.

Khurshudyan reported from Kyiv. Siobhán O’Grady in Kyiv and Alex Horton in Washington contributed to this report.