Despite repeated pleas for a greenlight from Ukraine, Biden had long resisted calls for a change from within the US Congress and latterly from allies such as Britain.

But with the clock ticking down before the inauguration of Trump -- who has expressed admiration for Russian leader Vladimir Putin, and vowed to end the fighting in Ukraine in "one day" -- on January 20, Biden has given permission for the powerful Army Tactical Missile System (ATACMS) to target Russian territory.

The New York Times and The Washington Post, which first reported the news, said the 81-year-old Democrat's shift came in response to North Korean troops being deployed to help Moscow's war effort against its neighbor.

Previously, US officials had worried about the danger of escalating the conflict with nuclear-armed Russia, as well as the risk of depleting Washington's own stocks of the valuable munitions.

Ukrainian President Volodymyr Zelensky had pleaded again for restrictions to be lifted on Friday.

"We see every site where Russia is amassing these North Korean soldiers on its territory  -- all their camps. We could strike preventively, if we had the ability to strike long enough," Zelensky said in an evening address.

On Sunday, he noted that the reports about Biden's policy change had not been openly confirmed by the White House but said the "missiles will speak for themselves." 

Putin has warned that the use of ATACMS inside Russia would mean the NATO alliance being "at war" with his country -- a threat he has made previously when Ukraine's Western backers have escalated their military assistance.

Western intelligence reports suggest that around 10,000 North Korean troops are being deployed in Russia. 

The American shift is likely to lead European allies to review their stances on long-range missiles, even though experts say the weapons alone will not decisively tilt current battlefield dynamics.

France and Britain have provided Ukraine with their long-range Storm Shadow and SCALP missiles, but have held back from authorizing their use inside Russia without American approval for ATACMS.

During a meeting with French leader Emmanuel Macron on Monday, Prime Minister Keir Starmer of Britain discussed how to put Ukraine in "the strongest possible position going into the winter," his office said afterward.

Speaking in Argentina on Sunday, Macron said Putin "does not want peace" and that "it's clear that President Putin intends to intensify the fighting."

German Chancellor Olaf Scholz has refused to supply Ukraine with his country's Taurus missiles with a range of over 310 miles (500 kilometers) over fears that they could hit Russian territory.

In the final two months of the Biden administration, US officials have promised to spend the remaining $6 billion of approved Ukraine funding at their disposal.

It is unclear how many ATACMS remain in Ukraine's arsenal.

The missile is made by US defense group Lockheed Martin and has a top range of 190 miles (300 kilometers).

Trump has provided no specifics on how he plans to end the conflict in Ukraine, and is expected to pressure Kyiv to give up its claim to some or all of its territory conquered by Russia.

Trump's pick for his National Security Advisor, Mike Waltz, said recently that "pouring more billions in (to Ukraine) is the definition of insanity at this point," adding that the conflict was a "stalemate."

Trump's eldest son, Donald Trump Jr., has even mocked Zelensky online, sharing a clip on Instagram last weekend that said the Ukrainian leader was just weeks away "from losing your allowance."

Writing Sunday about Biden's missile decision, he said "the military industrial complex seems to want to make sure they get World War 3 going before my father has a chance to create peace and save lives."

Russia pounded Ukraine on Sunday with a massive attack that killed 11 civilians across several regions and damaged the country's already fragile energy grid.