President Joe Biden has authorised Ukraine to use US-supplied missiles to strike deeper inside Russia, easing limitations on the longer range weapons as Russia deploys thousands of North Korean troops to reinforce its war, according to a US 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 US 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 US 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, 40 kilometers (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.

The comment appeared to be a dig at German Chancellor Olaf Scholz, who spoke Friday with Putin in the first such call with a sitting head of a major Western power in nearly two years.

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 US and other NATO members into direct conflict with nuclear-armed Russia.

Putin has warned that Moscow could provide long-range weapons to others to strike Western targets if NATO allies allow Ukraine to use their arms to attack Russian territory.

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, UN Secretary-General Antonio Guterres told reporters that the UN position is “to avoid a permanent deterioration of the war in Ukraine.”

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

The longer-range missiles are likely to be used in response to North Korea’s decision to support Putin’s invasion of Ukraine, according to one of the people familiar with the development.

The overall supply of ATACMS missiles is short, so US officials in the past have questioned whether they could give Ukraine enough to make a difference. Some supporters of Ukraine say that even a few strikes deeper inside Russia would force its military to change deployments and expend more of its resources.

North Korea has provided thousands of troops to Russia to help Moscow try to claw back land in the Kursk border region that Ukraine seized this year. The introduction of North Korean troops to the conflict comes as Moscow has seen a favorable shift in momentum. Trump has signaled that he could push Ukraine to agree to give up some land seized by Russia to find an end to the conflict.

As many as 12,000 North Korean troops have been sent to Russia, according to US, South Korean and Ukrainian assessments. US and South Korean intelligence officials say North Korea also has provided Russia with significant amounts of munitions to replenish its dwindling weapons stockpiles.

Trump, who takes office in January, spoke for months as a candidate about wanting Russia’s war in Ukraine to be over, but he mostly ducked questions about whether he wanted U.S. ally Ukraine to win.

He also repeatedly slammed the Biden administration for giving Kyiv tens of billions of dollars in aid. His victory has Ukraine’s international backers worrying that any rushed settlement would mostly benefit Putin.

America is Ukraine’s most valuable ally in the war, providing more than $56.2 billion in security assistance since Russian forces invaded in February 2022.

Worried about Russia’s response, however, the Biden administration repeatedly has delayed providing some specific advanced weapons sought by Ukraine, agreeing only under pressure from Kyiv, its supporters and in consultation with allies.

That includes initially refusing Zelenskyy’s pleas for advanced tanks, Patriot air defense systems and F-16 fighter jets, among other systems.

The White House agreed in May to allow Ukraine to use US-provided weaponry for limited strikes just across the border with Russia.