President Joe Biden has authorised Ukraine's use of US-supplied long-range missiles to strike inside Russia for the first time, according to reports.
The decision would allow Kyiv to conduct strikes deeper into Russia and would mark a major US policy shift, just 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.
Ukrainian President Volodymyr Zelensky said on Sunday the missiles would "speak for themselves".
Two US officials and a source familiar with the situation revealed the significant change in Washington's policy in the Ukraine-Russia conflict on Sunday, news agency Reuters reported.
Mr Zelensky later said in his Sunday evening address: "Today, many in the media are saying that we have received permission to take appropriate actions. But strikes are not made with words.
“Such things are not announced. The missiles will speak for themselves.”
The weapons are likely to be used in response to North Korea's decision to send thousands of troops to Russia in support of Russian President Vladimir Putin's invasion of Ukraine, according to one source, AP said.
Mr Zelensky and many of his Western supporters have been pressing 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.
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.
As many as 12,000 North Korean troops have been sent to Russia, according to US, South Korean and Ukrainian assessments. US and South Korean intelligence officials say North Korea also has provided Russia with significant amounts of munitions to replenish its dwindling weapons stockpiles.
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 Putin.
News of Biden’s alleged approval of the missiles came hours after Russia carried out its largest air strike on Ukraine in almost three months.
In the early hours of Sunday, Russia launched around 120 missiles and 90 drones in a "massive" combined air strike on Ukraine's energy infrastructure that killed at least seven people, Ukrainian authorities said.
“These attacks again highlight Ukraine's need for additional air defence systems from our allies," Mr Zelensky said at the time.
