The United States provided Ukraine with intelligence that helped Kyiv attack and sink the flagship of Russia’s Black Sea fleet, the Moskva, in one of the most dramatic battlefield successes of the 71-day-old war, according to people familiar with the matter.

The missile strike by Ukrainian forces in April, an extraordinary embarrassment for the Kremlin that deprived Russia of a key vessel in its military campaign, may not have been possible without the U.S. assistance, these people said, underscoring how deeply Washington has become enmeshed in Ukraine’s fight against Russia. It is unclear how many Russian sailors died in the attack, but U.S. officials believe there were significant casualties.

Despite providing intelligence on the Moskva, the United States had “no prior awareness” of Ukraine’s decision to strike the warship, a U.S. official said. The official noted that the U.S. government shares maritime awareness with Ukraine to help the nation defend against threats. Russian vessels in the Black Sea have been launching missiles at Ukraine — and could be used to support an amphibious assault on the country, the official said.

Military analysts and experts have praised the Ukrainian military’s strength and ingenuity as it repels a larger Russian force that many believed was more sophisticated and technically superior. But the U.S. intelligence has also given the Ukrainian forces a significant advantage, allowing them to locate Russian forces, equipment, and command and control centers.

Absent the intelligence from the United States, Ukraine would have struggled to target the warship with the confidence necessary to expend two valuable Neptune missiles, which were in short supply, according to the people familiar with the strike, who spoke on the condition of anonymity to discuss sensitive intelligence. NBC News first reported that the United States had provided intelligence on the Moskva.

Since before the war began, the Biden administration has treated the issue of intelligence-sharing with Ukraine as extremely sensitive. Officials have insisted they only provide assistance that helps Ukraine defend itself, worried that Russia could view the provision of information used in attacks as a justification for retaliating directly against the United States and its allies.

“Ukraine combines information that we and others provide with intelligence they’re gathering themselves . . . and then they make their own decisions,” Pentagon spokesman John Kirby said Thursday in response to a New York Times report that U.S. intelligence had helped Ukraine target Russian generals in the field.

“We do provide them useful intelligence, timely intelligence,” Kirby added, but he did not detail what that is or how it has been exploited.

To avoid sparking a wider war between two nuclear-armed superpowers, President Joe Biden has ruled out sending U.S. troops to Ukraine, enforcing a no-fly zone over the country or providing certain categories of weapons, such as fighter planes, that could allow Ukraine to strike inside Russia.

“The [United States] provides battlefield intelligence to help the Ukrainians defend their country,” National Security Council spokesperson Adrienne Watson said in a statement Tuesday. “We do not provide intelligence with the intent to kill Russian generals.”

That is a legal distinction that may make little practical difference to Russian leaders. Kremlin spokesman Dmitry Peskov said Thursday that the Russian military is “well aware that the United States, Great Britain and NATO as a whole are constantly transmitting intelligence and other parameters to the Ukrainian armed forces.”

“This is well known, and of course, coupled with the flow of weapons that these countries and the alliance are sending to Ukraine — these are all actions that, let’s say, are not enabling a speedy completion of the operation,” said Peskov, who added that the West nevertheless wasn’t capable of preventing Russia from achieving its stated goals.

Asked if Russia would take specific measures in reaction to any intelligence-sharing that led to the deaths of Russian generals, Peskov said, “Of course, Russian forces are doing everything necessary in this situation,” according to state news agency RIA Novosti.

The intelligence-sharing with Ukraine differs from past conflicts in which the United States has worked directly with allies to execute strikes. During the U.S.-led surge in Iraq in 2008, for instance, the U.S. military partnered with Iraqi forces to locate and attack insurgents and militants using cellular phone data tracked by the National Security Agency. And in counterterrorism operations in Africa, the intelligence community has provided security services with the ability to track militants’ cellphones, for the purpose of trying to capture or kill them.

From the U.S. perspective, if Ukraine receives intelligence and then decides to take action in the country’s defense, the United States did not provide “targeting” information that directed the Ukrainians whom or what to strike.

Kirby seemed to draw that distinction last month when asked during a news conference whether a U.S. Navy patrol plane in the Black Sea region “was tracking the Moskva before it was attacked by Ukraine and provided them targeting information.”

Kirby said that the type of aircraft in question, P-8 Poseidons from a U.S. base in Italy, were used as part of NATO’s “air policing missions” over the area. “There was no provision of targeting information by any United States Navy P-8 flying in these air policing missions,” he said. He did not elaborate.

As the war has raged on, the United States has increased the volume and speed of the intelligence it is providing, according to U.S. and Ukrainian officials. In the early days of the conflict, some Ukrainian officials complained that information about Russian troop positions was slow to arrive and not precise enough to help the Ukrainians launch attacks.

Today the intelligence is flowing in “real time” and has proved to be a key enabler of the Ukrainian campaign, one senior Ukrainian official said. The U.S. has provided Ukraine a large amount of satellite imagery and reports about the Russian military, some of which are based on intercepted communications, according to U.S. and European officials.

The United States also took steps to provide intelligence about Russian positions in the south and east of the country that hadn’t been provided before the invasion. Washington previously didn’t hand over that information because it could have helped Ukraine launch offensive attacks on Russian and separatist positions on Ukrainian territory that had been occupied since 2014, officials said.

Before the invasion, on Feb. 24, the Biden administration declassified and released publicly intelligence, including satellite images, that pointed to a massive buildup in Russian forces along Ukraine’s border in what appeared to be preparations for an attack.

The U.S. is not alone in helping Ukraine.

Baltic intelligence services have had an especially large role in helping Ukraine with information about the Russian military in the eastern part of the country, a European intelligence official said. The flow of assistance has been of growing importance in recent weeks as the bulk of Russia’s military effort shifts to the region.

The Baltic assistance has been focused mostly on intercepts of phone calls and other battlefield communications within the Russian military, although it also includes intelligence analysis based on the those countries’ deep experience with Russia’s military organization, strategy and planning, the official said. Russian officers and soldiers have been using unencrypted cellphones and walkie-talkies to communicate with each other, which allows outsiders both to listen in on the conversations and to use the signal location to pinpoint the soldiers for strikes.

The Baltic intelligence contribution complements assistance the United States has been offering Ukraine, which has been more focused on southern Ukraine and the Black Sea region, the European official said.

Ukraine invested in the domestic development of anti-ship missiles after Russia annexed Crimea in 2014 and fueled a separatist war in the country’s eastern Donbas region.

The Ukrainians have an extremely limited supply, however. Testing of the Neptune went on for years, but the Luch design bureau, which manufactures the weapon in Ukraine, was only due to supply the first division of Neptunes to Ukrainian forces this year.

The target the Ukrainians decided to use the missiles against was loaded with symbolism.

Launched in 1979, the Moskva was one of few guided missile cruisers in the Russian Navy’s possession. Originally named the Slava, the vessel hosted Soviet leader Mikhail Gorbachev during his Malta summit with President George H.W. Bush in December 1989.

Russia initially attempted to conceal the Ukrainian strike on the Moskva. The Russian military said a fire had broken out aboard the ship, forcing the crew to evacuate. Later, Russian authorities said the ship sank during a storm while being towed back to port.

The Moskva played a role in the initial days of the war against Ukraine.

“I am a Russian warship,” the Moskva’s loudspeaker said to a group of Ukrainian border guards stationed on Snake Island in the Black Sea, demanding the Ukrainians lay down their arms and surrender. The Ukrainians responded, “Russian warship, go f--- yourself,” a phrase that subsequently became a mantra of Ukrainian resistance to the Russian invasion.

Initially, Ukrainian President Volodymyr Zelensky said the Ukrainian border guards had been fired upon and killed, but Ukrainian authorities later announced they may be alive and imprisoned by Russia in Sevastopol.

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The Washington Post’s John Hudson and Karoun Demirjian contributed reporting.