The White House and Pentagon declined to comment. Ukraine’s presidential office declined to comment.

Until recently, the Biden administration was steadfastly opposed to Ukraine firing ATACMS into Russian territory, warning that the measure could lead to escalation by the Kremlin that was out of proportion to its battlefield benefits.

ATACMS — pronounced “attack-ems” — is a supersonic guided missile system that can be fitted with either cluster munitions or conventional warheads, with a maximum range of about 190 miles. Ukraine for months has sought permission to use the powerful missiles against Russian territory, arguing that the weapons would enable its strapped forces to strike deep in the country and hit targets that would degrade the Kremlin’s war machine.

The arrival of the North Koreans in October in the Kursk region, where Ukraine launched a surprise offensive in August, was seen by the West as a major escalation and spurred an intense effort inside the Biden administration and with allies on how to respond.

The White House wants to put Ukraine in the best possible place ahead of peace talks that the new U.S. president is expected to spearhead early in his term, U.S. officials said. Even before the election, Biden had committed to surging aid to Ukraine in an effort to cement his legacy on his way out of office.

“President Biden has committed to making sure that every dollar we have at our disposal will be pushed out the door between now and January 20th,” Secretary of State Antony Blinken told reporters Wednesday in Brussels, where he was meeting with European counterparts to discuss how to support Ukraine in the wake of Trump’s win.

A second U.S. official said that Biden’s approval of ATACMS “is going to have a very specific and limited effect” on the battlefield, designed to limit concerns about escalation.

“If news of the policy shift is true,” said Michael Kofman, a Russian and Ukrainian military expert with the Carnegie Endowment for International Peace, “then it could be of operational benefit to Ukraine, enabling them to better defend and hold on to the territory they currently occupy in Kursk and help offset the benefit that Russia enjoys from employing North Korean forces in this specific part of the front.”

Previous steps framed as limited have cracked the door to wider forms of military assistance over the course of the nearly three-year war.

Russian President Vladimir Putin is “testing the West, NATO, and even South Korea, observing their response to North Korean forces joining his campaign,” Ukrainian President Volodymyr Zelensky said on X late last month. “If the response is weak, we should expect the numbers of foreign soldiers on our soil to increase.”

Russia’s capture of eastern Ukrainian territory has accelerated, buoying spirits inside the Kremlin, whose leaders now feel they have the advantage in a war that is no longer a stalemate.

The authorization follows months of resistance by the Biden administration about allowing Ukraine to use the ATACMS to hit targets within Russia. Senior U.S. officials have repeatedly expressed private concern that Russia could retaliate by escalating inside Ukraine and around the world. In denying Kyiv’s pleas to be able to fire ATACMS inside Russia, administration officials have publicly said that the use of the weapon would have marginal utility on the battlefield.

Pentagon officials, who were by far the most skeptical voice inside the administration, have argued that the benefits of allowing strikes in Russia would be limited because the Kremlin, anticipating a potential easing of the restraint, earlier this year pulled most of its warplanes and other assets deeper into Russia and out of range.

As of September, 90 percent of the Russian aircraft launching glide bombs into Ukraine were flying from airfields outside ATACMS range, Pentagon spokeswoman Sabrina Singh said at the time.

The policy move comes at a time of heightened political sensitivity as Biden seeks to alter Ukraine’s fortunes before Trump takes office, and as North Korean troops have bolstered Russia’s advantage on the battlefield.

Ukraine’s control of Russian territory has taken on intense significance as both sides scramble for advantage ahead of potential talks. People close to the Kremlin say that Putin is unwilling to start any negotiations while Ukrainians are on Russian soil. The Biden administration is focused on helping Kyiv preserve its bargaining leverage there as long as possible.

U.S. and Ukrainian officials believe that the presence of North Korean troops will free Russian forces to focus on gaining ground elsewhere as well as push the front lines forward in Kursk, where Ukraine captured territory in August, providing a morale boost to Ukrainians, who have been sapped by nearly three years of war. Pyongyang’s involvement has rattled Washington and its allies, who are wary of the assistance Putin might offer Kim in return.

At a summit of Asia-Pacific leaders in Peru on Friday, Biden met with Japanese Prime Minister Shigeru Ishiba and South Korean President Yoon Suk Yeol. In a statement, the three leaders said they “strongly condemn” North Korea’s troop deployment to Russia to “dangerously expand Russia’s war of aggression against Ukraine.”

The trio also noted the deepening military cooperation between the two countries, calling the supply of munitions and ballistic missiles “particularly egregious” given Russia’s status as a permanent member of the U.N. Security Council.

U.S. officials have said that their concerns about Russian escalation in response to Western military aid have diminished over time as one weapons system after another has been provided to Ukraine without significant retaliation in response. Ukraine is already using U.S. equipment inside Kursk to attack Russia.

But Putin has been explicit that he considers the use of ATACMS a red line. In September, he declared that a strike by the missiles into Russian territory, which would probably involve U.S. targeting assistance, “changes the very essence, the nature of the conflict,” warning that his country would retaliate.

Later that month, he revised Russia’s nuclear doctrine in what was interpreted as a veiled threat against the use of U.S.-provided long-range weapons on Russian soil.

Administration officials who have previously been skeptical of allowing Ukraine to use U.S. long-range weapons for strikes in Russia have said that given the limited number of the advanced missiles, the blowback may not be worth the potential battlefield advantage. But with North Korea’s increasing involvement in the conflict, the U.S. calculus appears to have shifted.

Officials characterized the decision as a limited evolution rather than a new chapter in the war.

The authorization for the use of ATACMS on targets within Russian territory follows repeated requests by Ukraine. Early this year, Kyiv asked Washington to provide long-range ATACMS and in August requested that its forces be allowed to use them in Kursk.

“We have adapted and adjusted to the needs of Ukraine as the battlefield changes, as what Russia is doing changes, as new elements are introduced — for example, the North Korean forces,” Blinken said during the visit to Brussels on Wednesday.

“I can tell you that we will continue to adapt and adjust again, to make sure that Ukraine is in the strongest possible position to deal with this aggression,” Blinken said. He declined to comment on specifics about the steps the Biden administration was taking to respond to the North Korean troops.

If North Korean soldiers “do deploy to fight against Ukraine, they’re fair game. They’re fair targets,” White House spokesman John Kirby said last month, warning that anyone fighting Ukrainian forces would face retaliation from Kyiv. “The Ukrainian military will defend themselves against North Korean soldiers the same way they’re defending themselves against Russian soldiers.”

Trump is expected to be far more skeptical of U.S. aid for Ukraine than Biden has been, and he has expressed eagerness to broker a peace deal between Moscow and Kyiv. Putin and Trump spoke in a call after the election, according to five people familiar with the matter who spoke on the condition of anonymity to discuss a sensitive diplomatic exchange. In that call, several people said, Trump warned the Russian leader not to escalate in Ukraine and said he wanted to discuss the resolution of the war soon.

The Kremlin denied that the call took place.

Biden, though he has authorized tens of billions of dollars in military aid to Ukraine, has been reluctant to grant Kyiv advanced U.S. weapons. He hesitated about sending the Patriot air defense system, then relented. A similar policy evolution saw Washington initially refuse to give Ukraine U.S.-made Abrams tanks and F-16 fighter jets.

The White House in May reversed a broad ban on Ukraine using U.S. military assistance to strike within Russia, after the Kremlin took advantage of the restriction by concentrating its forces in border regions and attacking across the frontier with relative impunity.

When Biden finally authorized the longer-range ATACMS earlier this year, he limited their use to within Ukraine’s own territory, enabling them to strike Russian forces on the Crimean Peninsula but not to hit within Russia itself..

The White House had maintained its ban on ATACMS strikes in Russia in part because of concerns that Russia would respond with force against U.S. and allies’ interests elsewhere. That could include the use of even more devastating weapons inside Ukraine, an increase in sabotage attacks in Europe and the United States, or intensified support for Iran and for the Houthi rebels in Yemen who have snarled global shipping, two other senior administration officials said in September.

Though this policy reversal gives Kyiv a significant new tool, Biden administration officials note that Ukraine has very limited stocks of ATACMS. Russia has shown that it has a significant shoot-down capability, and the Pentagon, whose own missile supply is dwindling, says it does not have many more to give without affecting U.S. readiness.

Defenders of Biden’s approach say he has been managing risks of escalation amid periods in which U.S. intelligence assessments have offered real warnings about the possibility of Putin using a nuclear weapon against Ukraine.

But the halting provision of advanced weapons and other cautious policies have caused frustration in Kyiv, Ukrainian officials have said. When troops finally receive the weapons or are freed to use them, the military returns are often diminished because conditions on the battlefield have changed, leading to preventable casualties and setbacks, according to soldiers and commanders on the ground.

Siobhán O’Grady in Kyiv, Matt Viser in Lima, Peru, and Michelle Ye Hee Lee in Seoul contributed to this report.