The Kremlin warned Monday that President Joe Biden’s decision to let Ukraine strike targets inside Russia with U.S.-supplied longer-range missiles adds “fuel to the fire” of the war and would escalate international tensions even higher.

Biden’s shift in policy added an uncertain, new factor to the conflict on the eve of the 1,000-day milestone since Russia began its full-scale invasion in 2022.

It also came as a Russian ballistic missile with cluster munitions struck a residential area of Sumy in northern Ukraine, killing 11 people, including two children, and injuring 84 others. Another missile barrage sparked apartment fires in the southern port of Odesa, killing at least 10 people and injuring 43, including a child, Ukraine’s Interior Ministry said.

Washington is easing limits on what Ukraine can strike with its American-made Army Tactical Missile System, or ATACMs, U.S. officials told The Associated Press on Sunday, after months of ruling out such a move over fears of escalating the conflict and bringing about a direct confrontation between Russia and NATO.

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The Kremlin was swift in its condemnation.

“It is obvious that the outgoing administration in Washington intends to take steps and they have been talking about this, to continue adding fuel to the fire and provoking further escalation of tensions around this conflict,” spokesman Dmitry Peskov told reporters.

The scope of the new firing guidelines isn’t clear. But the change came after the U.S., South Korea and NATO said recently that North Korean troops are in Russia and apparently are being deployed to help Moscow drive Ukrainian troops out of Russia’s Kursk border region.

Russia also is slowly pushing Ukraine’s outnumbered army backward in the eastern Donetsk region. It has also conducted a devastating aerial campaign against civilian areas in Ukraine.

Peskov referred journalists to a statement made by President Vladimir Putin in September in which he said allowing Ukraine to target Russia would significantly raise the stakes.

It would change “the very nature of the conflict dramatically,” Putin said at the time. “This will mean that NATO countries — the United States and European countries — are at war with Russia.”

Peskov claimed that Western countries supplying longer-range weapons also provide targeting services to Kyiv. “This fundamentally changes the modality of their involvement in the conflict,” he said.

Putin warned in June that Moscow could provide longer-range weapons to others to strike Western targets if NATO allowed Ukraine to use its allies' arms to attack Russian territory. He also reaffirmed Moscow’s readiness to use nuclear weapons if it sees a threat to its sovereignty.

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President-elect Donald Trump, who takes office in about two months, has raised uncertainty about whether his administration would continue vital military support to Ukraine. He has also vowed to quickly end the war.

Ukrainian President Volodymyr Zelenskyy gave a muted response to the approval that he and his government have been requesting of Biden for more than a year.

“Today, much is being said in the media about us receiving permission for the relevant actions,” Zelenskyy said in his nightly video address Sunday.

“But strikes are not made with words. Such things are not announced. The missiles will speak for themselves,” he said.

The new policy's consequences on the battlefield are uncertain. ATACMS, which have a range of about 300 kilometers (190 miles), can reach targets far behind the about 1,000-kilometer (600-mile) front line in Ukraine, but they have relatively short range compared with other types of ballistic and cruise missiles.

The policy change came “too late to have a major strategic effect,” said Patrick Bury, a senior associate professor in security at the University of Bath in the United Kingdom.

“The ultimate kind of impact it will have is to probably slow down the tempo of the Russian offensives which are now happening,” he said.

Ukraine could target enemy troops concentrated in Kursk or logistics hubs or command headquarters, Bury added.

The cue for the policy change was the arrival in Russia of North Korean troops, according to Glib Voloskyi, an analyst at the CBA Initiatives Center, a Kyiv-based think tank.

“This is a signal the Biden administration is sending to North Korea and Russia, indicating that the decision to involve North Korean units has crossed a red line,” he told AP.

Russian lawmakers and state media bashed the West over what they called an escalatory step, and threatened a harsh response.

“Biden, apparently, decided to end his presidential term and go down in history as ‘Bloody Joe,’” senior lawmaker Leonid Slutsky told Russian state news agency RIA Novosti.

Vladimir Dzhabarov, deputy head of the foreign affairs committee in the upper house of Russian parliament, called it "a very big step toward the start of World War III” and an attempt to “reduce the degree of freedom for Trump.”

Russian newspapers offered similar predictions of doom. “The madmen who are drawing NATO into a direct conflict with our country may soon be in great pain,” Rossiyskaya Gazeta said.

Some NATO allies welcomed the move.

President Andrzej Duda of Poland, which borders Ukraine, praised the decision as “much needed” and calling it a “very important, maybe even a breakthrough moment“ in the war.

“In the recent days, we have seen the decisive intensification of Russian attacks on Ukraine, above all, those missile attacks where civilian objects are attacked, where people are killed, ordinary Ukrainians,” Duda said.

Foreign Minister Margus Tsahkna of Russian neighbor Estonia said easing restrictions on Ukraine was “a good thing.”

“We have been saying that from the beginning — that no restrictions must be put on the military support,” he said at a meeting of senior European Union diplomats in Brussels. “And we need to understand that situation is more serious (than) it was even maybe like a couple of months ago.”

Lithuanian Foreign Minister Gabrielius Landsbergis said he’s not “opening the champagne” yet as it is unclear exactly what restrictions have been lifted and whether Ukraine has enough of the U.S. weapons to make a difference.