The decision draws Biden even deeper into a war in which Russian President Vladimir Putin has repeatedly raised the prospect of a nuclear strike, a concern for a U.S. leader who matured amid the U.S.-Soviet nuclear confrontations of the 1960s. Biden has been cautious about escalation — but also mindful that the Ukrainians have repeatedly been granted greater capabilities and faced down a Kremlin that did little in response.

A growing number of the United States’ European allies in recent days also had urged the administration to lift its opposition, signaling an intent to allow their own weapons to be used against military targets on Russian soil. Although Ukraine has used some European arms as well as their own to fight back, Washington’s say-so has been the most important because of the quantity and the quality of its equipment.

The shift allows Ukraine to use U.S.-provided artillery and rocket launchers to hit Russian troops and equipment just across the border from Kharkiv and to strike missiles headed toward Ukrainian territory, U.S. officials said. They emphasized that the Biden administration’s policy barring longer-range strikes inside Russia “has not changed.”

“This is in response to the Ukrainian request, which was to be able to respond to attacks that are emanating from that area” in Russia’s Belgorod region, “to be able to hit Russian forces and arms depots,” one U.S. official said, adding that “Ukraine is not asking for a blanket policy change, and we’re not changing that policy.”

The Russian Embassy in Washington did not immediately respond to a request for comment. Speaking earlier Thursday, Dmitry Peskov, the Kremlin’s chief spokesman, chastised the United States and its NATO allies, saying the alliance was responsible for setting off “a new round of escalating tension.”

“They are doing this deliberately,” Peskov said. “We are hearing a lot of belligerent statements.”

Biden’s reassessment, first reported by Politico, was several weeks in the making. It is a byproduct, officials said, of Russia’s renewed cross-border assault on Kharkiv, the mounting pressure from across Europe and a visit to Kyiv this month by U.S. Secretary of State Antony Blinken that reinforced the peril facing Ukraine after more than two years of war.

Ukraine had been holding off Russian forces with increasing difficulty in recent months as U.S. military aid all but dried up last fall after congressional Republicans came out in opposition to further aid. Kyiv faced dwindling stocks of ammunition and antiaircraft missiles — to the point where diplomats posted in the Ukrainian capital started to worry this spring about a sudden collapse on the front lines and a major Ukrainian defeat.

That changed after Congress approved aid last month. But Ukrainian morale remains low, and shortages of trained soldiers mean that the front lines are still vulnerable despite the resumption of U.S. military assistance. Russia, meanwhile, has taken advantage of the moment, driving against Kharkiv and throughout other parts of the long front line as it tries to exploit the window before more U.S. aid helps stabilize Kyiv’s troops.

Ukrainian officials first asked the White House for permission weeks ago, on May 13, days after the assault on Kharkiv began and the day before Blinken arrived in Kyiv, another U.S. official said. National security adviser Jake Sullivan, Defense Secretary Lloyd Austin and Chairman of the Joint Chiefs of Staff Gen. Charles Q. Brown Jr. agreed to recommend a policy shift to Biden, the official said.

Sullivan took the recommendation to the president two days later, as Blinken was departing Kyiv, and Biden agreed to it that same day, May 15, this official said. Blinken, meeting Biden later that week after his Ukraine trip, agreed that the shift made sense, the official said, adding that Biden asked for top officials to work through the details and the risks before a final approval.

Biden signed off on the change several days ago, and the policy went info effect Thursday.

Blinken, during a visit to Moldova this week, became the first senior Biden administration official to publicly indicate that Washington was considering the policy shift, telling reporters that “as the battlefield has changed, as what Russia does has changed in terms of how it’s pursuing its aggression, escalation, we’ve adapted and adjusted, too.”

Blinken emerged from his trip to Kyiv this month convinced that some form of limited policy shift was necessary, officials said. At a closed-door meeting of NATO foreign ministers in Prague on Thursday, the top U.S. diplomat hinted at an impending policy shift but did not provide details, one participant said. When he was asked directly about it at one point, he smiled but stayed silent, the person said.

NATO Secretary General Jens Stoltenberg said in a recent interview with the Economist that the time had come for allies to rethink their restrictions. “Especially now when a lot of the fighting is going on in Kharkiv, close to the border, to deny Ukraine the possibility of using these weapons against legitimate military targets on Russian territory makes it very hard for them to defend themselves.”

In the days since, allies including France, the Netherlands, Canada and Finland echoed the sentiment.

In a visit to Kyiv this month, British Foreign Secretary David Cameron said Ukraine has the right to use London-provided weapons to strike targets in Russia. “Just as Russia is striking inside Ukraine, you can quite understand why Ukraine feels the need to make sure it’s defending itself.”

Although Moscow claims that five regions of Ukraine, including Crimea, are Russian territory, it has been highly sensitive to the increasing calls to allow Ukraine to use Western weapons to strike military targets within Russia itself. Putin earlier this week warned this could lead to “serious consequences.”

In a sign of the Kremlin’s anxiety, Putin hinted that Russia could use nuclear strikes against small European nations if NATO allowed Ukraine to attack what he called “deep in Russian territory.” He warned that NATO officials “should be fully aware of what is at stake.”

“If Europe were to face those serious consequences, what will the United States do, considering our strategic arms parity? It is hard to tell,” he said, referring to U.S. and Russian nuclear arsenals. “Are they looking for a global conflict?”

Under pressure from Kyiv and European allies, Biden’s risk appetite has changed repeatedly over the course of the war when he decided to expand Ukraine’s arsenal with Stinger missiles, HIMARS launchers, advanced missile defense systems, drones, helicopters, M1 Abrams tanks and fighter jets.

Amid the restrictions on U.S. support for cross-border attacks, Ukraine has been using its own long-range attack drones to hit Russian civilian and military targets. But those aircraft have payload limitations and are not as effective.

U.S. officials remain concerned about Ukrainian cross-border attacks on Russian territory, including the targeting of oil refineries and nuclear early–warning systems, fearing that they could dangerously unsettle Moscow. Washington conveyed its concerns to Kyiv about two attempted attacks over the past 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 Krasnodar region, appeared to have caused some damage.

Russia’s advances have also spurred discussion between allies about sending military trainers to Ukraine — another move long seen as potentially escalatory. But conditions on the battlefield seem to have convinced some allies that it makes sense to take the training closer to Ukraine’s troops, allowing them to move more quickly and easily to the front line afterward.

In February, French President Emmanuel Macron surprised many by suggesting that “nothing should be ruled out” when it comes to sending trainers to Ukraine, but he did not offer concrete details.

Ukraine’s top general, Oleksandr Syrsky, announced this week that Ukraine and France had signed an agreement for French soldiers to train troops on Ukrainian soil, then quickly walked it back, saying the issue was still up for discussion.

But French officials pointedly did not deny that talks were advancing, leading to speculation that an announcement about some sort of training mission could come soon.

Any training, NATO diplomats stressed, would be organized between member states and Ukraine bilaterally, not by NATO itself, which has kept an official distance from the war.

Biden has long ruled out sending U.S. troops to Ukraine. Whether that prohibition falls by the wayside like his other red lines remains to be seen.

Hudson and Rauhala reported from Prague. Robyn Dixon in Riga, Latvia, and Alex Horton, Tyler Pager and Dan Lamothe in Washington contributed to this report.