KYIV, Ukraine — The Kremlin on Monday warned 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. 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 said 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.

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 deployed to help Moscow drive Ukrainian troops from Russia’s Kursk 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 annual 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.

“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 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 had 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.

Consequences of the new policy are uncertain. ATACMS, which have a range of about 190 miles, can reach far behind the 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, England.

“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 that the U.S. move would not alter the war’s course. 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.

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.