WASHINGTON— President Biden has for the first time authorized Ukrainian forces to use Western-made long-range weapons to strike inside Russia, allowing Kyiv to better defend itself against Russia , according to U.S. officials.

That means Ukraine could initially target positions in the Kursk region, where Russia has amassed more than 50,000 troops, including some  10,000 soldiers from North Korea , in an effort to recapture the territory. Ukraine’s forces seized the territory earlier this year.

The introduction of thousands of North Korean troops onto the battlefield and the expected Kursk operation led to a change in Biden’s calculus, U.S. officials and other people familiar with the deliberations said.  Biden made the decision before he left Thursday for South America , one of his last foreign trips as president, said one of the people. The decision was reported earlier by the New York Times .

The relaxation of Ukraine’s use of the Army Tactical Missile System, or ATACMS, comes after Kyiv pleaded for months to be allowed to strike inside Russia. The restrictions gave Moscow the upper hand in the war, Ukrainian President Volodymyr Zelensky and his top aides said publicly and privately.

But Biden administration officials were loath to greenlight the strikes until now for fear it could cross a red line for Russia and provoke a wider conflict. Some American officials were also concerned that the U.S. didn’t have enough ATACMS in its own inventory to replenish what Ukraine would use.

The Ukrainian forces have been using drones for some deep strikes but believed that the ATACMS would be more effective. ATACMS, a surface-to-surface missile system fired from a mobile launcher vehicle, can strike between 100 and 190 miles away, depending on the model of the system, well behind Russian lines.

Zelensky briefly commented on reports of the lifting of restrictions during his evening address on Sunday, saying: “We don’t strike with words. Such things are not announced. Missiles will speak for themselves.”

Some U.S. and Western officials say the missiles are unlikely to alter the conflict much, given that Russia can simply move most of its war materiel farther away and in many cases has already done so.

Allowing Ukraine to strike inside Kursk is an important step, but it must extend beyond the region to achieve a significant effect, said George Barros, an analyst at the Institute for the Study of War who has compiled a map of hundreds military targets within ATACMS range.  undefined undefined “Incrementalism will not deliver decisive effects,” Barros said. He added that Kyiv would benefit greatly from being permitted to strike a range of targets such as brigade headquarters and logistical infrastructure over a large territory.

Sen. Roger Wicker (R-Miss.), the highest-ranking Republican on the Armed Services Committee, said he was encouraged by the reports on Biden’s decision before excoriating the president for failing to make the move earlier.

The policy shift appears to be part of a Biden administration push to give what it can to Ukraine before Jan. 20. The decision comes a week after the administration opted to allow American defense contractors into Ukraine for the first time to help fix Western weaponry and aircraft, including the American Patriot missile defense system and the F-16 jet fighter.

The administration is also scrambling to get more than $7 billion in weaponry to Ukraine before Biden leaves office,  fearing that the incoming administration will curtail Ukraine arms shipments.

Steven Cheung , communications director for President-elect Donald Trump , didn’t directly address the policy change. “As President Trump has said on the campaign trail, he is the only person who can bring both sides together in order to negotiate peace, and work towards ending the war and stopping the killing,” he said.

Trump and Vice President-elect JD Vance are expected to press for negotiations to end the conflict in Ukraine. European allies initially balked at the idea of peace talks, but a continued stalemate in the war has changed some leaders’ minds about supporting a conflict with no clear end.

Last week, German Chancellor Olaf Scholz called Russian President Vladimir Putin , the first known call between the autocrat and a Western leader in two years. It is a sign that Putin’s international isolation might be ending. Both Polish Prime Minister Donald Tusk and Zelensky criticized Scholz for holding the call.

Write to Gordon Lubold at gordon.lubold@wsj.com and Alexander Ward at alex.ward@wsj.com