President Joe Biden has authorised Ukraine’s use of US-supplied long-range missiles to strike inside Russia for the first time, according to AP sources.
The decision is a major US policy shift and comes as Mr Biden is about to leave office.
President-elect Donald Trump has pledged to limit American support for Ukraine and end the war as soon as possible.
The weapons are likely to be used in response to North Korea’s decision to send thousands of troops to Russia in support of President Vladimir Putin’s invasion of Ukraine, according to one source.
Ukrainian President Volodymyr Zelensky and many of his Western supporters have been pressing Mr Biden for months to allow Ukraine to strike military targets inside Russia with Western-supplied missiles, saying the US ban had made it impossible for Ukraine to try to stop Russian attacks on its cities and electrical grids.
Some supporters have argued that this and other US constraints could cost Ukraine the war.
The debate has become a source of disagreement among Ukraine’s Nato allies.
Mr Biden had remained opposed, determined to hold the line against any escalation that he felt could draw the US and other Nato members into direct conflict with Russia.
But North Korea has deployed thousands of troops to Russia to help Moscow try to claw back land in the Kursk border region that Ukraine seized this year.
The introduction of North Korean troops to the conflict comes as Moscow has seen a favourable shift in momentum.
Mr Trump has signalled that he could push Ukraine to agree to give up some land seized by Russia to find an end to the conflict.
As many as 12,000 North Korean troops have been sent to Russia, according to US, South Korean and Ukrainian assessments.
American and South Korean intelligence officials say North Korea also has provided Russia with significant amounts of munitions to replenish its dwindling weapons stockpiles.
Mr Trump, who takes office in January, spoke for months as a candidate about wanting Russia’s war in Ukraine to be over, but he mostly ducked questions about whether he wanted US ally Ukraine to win.
He also repeatedly slammed the Biden administration for giving Kyiv tens of billions of dollars in aid.
His election victory has Ukraine’s international backers worrying that any rushed settlement would mostly benefit Mr Putin.
America is Ukraine’s most valuable ally in the war, providing more than 56.2 billion dollars in security assistance since Russian forces invaded in February 2022.
Worried about Russia’s response, however, the Biden administration has repeatedly delayed providing some specific advanced weapons sought by Ukraine, only agreeing under pressure from Ukraine and in consultation with allies, after long denying such a request.
That includes initially refusing Mr Zelensky’s pleas for advanced tanks, Patriot air defence systems, F-16 fighter jets, among other systems.
The White House agreed in May to allow Ukraine to use the ATACMS missile system for limited strikes just across the border with Russia.
