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 and injuring 84 others. Another missile barrage sparked apartment fires in the southern port of Odesa, killing at least 10 people and injuring 43, 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.

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 said.

Russia’s U.N. Ambassador Vassily Nebenzia went further at a U.N. Security Council meeting marking 1,000 days of war, saying Moscow is “astounded” that the leaders of Britain and France “are eager to play into the hands of the exiting administration and are dragging not just their countries but the entire Europe into large-scale escalation with drastic consequences.”

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

Biden’s decision almost entirely was triggered by North Korea’s entry into the fight, according to a U.S. official who spoke on condition of anonymity to discuss internal deliberations, and was made just before he left for the Asia-Pacific Economic Cooperation summit in Peru.

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 from 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. After signing a treaty with North Korea, Putin issued an explicit threat to provide weapons to Pyongyang, noting Moscow could mirror Western arguments that it’s up to Ukraine to decide how to use them.

“The Westerners supply weapons to Ukraine and say: ‘We do not control anything here anymore and it does not matter how they are used,’” Putin has said. “Well, we can also say: ‘We supplied something to someone — and then we do not control anything.’ And let them think about it.”

Putin has also reaffirmed Moscow’s readiness to use nuclear weapons if it sees a threat to its sovereignty.

Biden’s move will “mean the direct involvement of the United States and its satellites in military action against Russia, as well as a radical change in the essence and nature of the conflict,” Russia’s Foreign Ministry said.

President-elect Donald Trump, who takes office Jan. 20, has raised uncertainty about whether his administration would continue military support to Ukraine. He has also vowed to end the war quickly.

Ukrainian President Volodymyr Zelenskyy gave a muted response Sunday to the approval that he and his government have request for over a year, adding: “The missiles will speak for themselves.”

“The longer Ukraine can strike, the shorter the war will be,” Ukrainian Foreign Minister Andrii Sybiha said Monday before the U.N. Security Council meeting marking the 1,000th-day milestone.

Asked whether the United Kingdom would follow the United States in authorizing use of its longer-range missiles, U.K. Foreign Secretary David Lammy, who chaired the meeting, declined to comment. He said doing so would risk “operational security and can only play into the hands of Putin.”

France’s U.N. Ambassador Nicolas De Riviere, whose country has also given Ukraine longer-range missiles, told the Security Council without directly saying what his country will do that “The right of Ukraine to its legitimate defense includes the possibility of striking military targets involved in operations aimed at the territory.”

Ukraine’s Sybiha said a green light from the U.S. to use longer-range missiles against Russia “could be a game changer,” but others are less certain.

ATACMS, which have a range of about 300 kilometers (190 miles), can reach 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, adding that Ukraine could strike targets in Kursk or logistics hubs or command headquarters.

Jennifer Kavanagh, director of military analysis at Defense Priorities, agreed the U.S. move would not alter the war’s course, noting Ukraine “would need large stockpiles of ATACMS, which it doesn’t have and won’t receive because the United States’ own supplies are limited.”

On a political level, the move “is a boost to the Ukrainians and it gives them a window of opportunity to try and show that they are still viable and worth supporting” as Trump prepares to take office, said Matthew Savill, director of Military Sciences at the Royal United Services Institute in London.

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 said.

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

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

Vladimir Dzhabarov, deputy head of the foreign affairs committee in the upper house of 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 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.

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

“We have been saying that from the beginning — that no restrictions must be put on the military support,” he told 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.”

But Slovakian Prime Minister Robert Fico, known for his pro-Russian views, described Biden’s decision as “an unprecedented escalation” that would prolong the war.