MANAUS, Brazil>> President Joe Biden has authorized Ukraine to use U.S.-supplied long-range missiles to strike deeper inside Russia, easing limitations on the weapons as Russia deploys thousands of North Korean troops to reinforce its war, according to a U.S. official and three other people familiar with the matter.

The decision allowing Kyiv to use the Army Tactical Missile System, or ATACMs, for attacks farther inside Russia comes as President Vladimir Putin positions North Korean troops along Ukraine’s northern border to try to reclaim hundreds of miles of territory seized by Ukrainian forces.

Biden’s move also follows the presidential election victory of Donald Trump, who has said he would bring about a swift end to the war and raised uncertainty about whether his administration would continue the United States’ vital military support for Ukraine.

The official and the others knowledgeable about the matter were not authorized to discuss the U.S. decision publicly and spoke on condition of anonymity.

Ukrainian President Volodymyr Zelenskyy’s reaction Sunday was notably restrained.

“Strikes are not made with words,” he said during his nightly video address. “Such things are not announced. The missiles will speak for themselves.”Zelenskyy and many of his Western supporters have been pressing Biden for months to allow Ukraine to strike military targets deeper inside Russia with Western-supplied missiles, saying the U.S. ban had made it impossible for Ukraine to try to stop Russian attacks on its cities and electrical grids.

Zelenskyy’s statement came shortly after he posted a message of condolence on Telegram following a Russian attack on a nine-story building that killed at least eight people in the northern city of Sumy, 24 miles from the border with Russia.

Russia also launched a massive drone and missile attack, described by officials as the largest in recent months, targeting energy infrastructure and killing civilians. The attack came as fears are mounting about Moscow’s intentions to devastate Ukraine’s power generation capacity before the winter. “And this is the answer to everyone who tried to achieve something with Putin through talks, phone calls, hugs and appeasement,” Zelenskyy said.

Some supporters have argued that the limitation and other U.S. constraints could cost Ukraine the war. The debate has become a source of disagreement among Ukraine’s NATO allies.

Biden had remained opposed, determined to hold the line against any escalation that he felt could draw the U.S. and other NATO members into direct conflict with nuclear-armed Russia.

News of Biden’s decision followed meetings over the last two days with the leaders of South Korea, Japan and China. The addition of North Korean troops was central to the talks, which took place on the sidelines of the Asia-Pacific Economic Cooperation summit in Peru.

Biden did not mention the decision during a speech at a stop in the Amazon rainforest in Brazil on his way to the Group of 20 summit.

Asked about the decision, U.N. Secretary-General Antonio Guterres said in a press conference that the body’s position is “to avoid a permanent deterioration of the war in Ukraine.”

“We want peace, we want fair peace,” Guterres said on Sunday before the summit in Rio de Janeiro. He didn’t elaborate.