President Joe Biden has permitted Ukraine to use long-range missiles supplied by the US to strike targets inside Russia, in a major foreign-policy about-face.
While the White House is yet to acknowledge the shift, Biden's counterpart in Ukraine, Volodymyr Zelenskyy, addressed it in his evening video update released late Sunday, local time, saying the missiles would "speak for themselves".
"Such things are not announced," he said.
Previously, the US had insisted the weapons it supplied Kyiv could only be used to attack Russian military assets occupying Ukrainian territory, with one exception that came earlier this year designed to protect the border city of Kharkiv.
It's understood the policy change will pave the way for Ukraine to use the Army Tactical Missile Systems, known colloquially as ATACMS on targets inside Russia.
ATACMS have a range of about 300 kilometres, meaning dozens of military assets like bases and storage facilities in Russia are now vulnerable.
The Kremlin has previously said it would consider Ukraine using Western weapons to attack targets in Russia as a major escalation in the war.
Vladimir Dzhabarov, an upper house MP, was quoted in the Russian state news agency TASS late Sunday as saying: "This is a very big step towards the start of World War Three."
Biden's decision comes shortly after it was revealed North Korean troops had been sent to bolster Putin's forces.
If and when Ukraine makes use of the looser parameters, it would be the first time since Russia's full-scale invasion long-range missiles had been sent over the border.
It's not, however, the first time Kyiv has attacked targets in Russia with US-supplied weapons.
In May, Biden permitted Zelenskyy's military to use the High Mobility Artillery Rocket System (HIMARS) — which has a range of about 80km — to strike targets in Russia near Kharkiv, which is Ukraine's second-largest city.
The change in foreign policy comes as Biden prepares to leave the White House in January.
Donald Trump, who won the US presidential election held earlier this month, is preparing to take over, and has promised to bring the war in Ukraine to an end as soon as possible. He hasn't said how he'll do that.
In the days after his victory, Trump and Zelenskyy spoke via telephone.
Russia's full-scale invasion of Ukraine began in February 2022.
Since then, it's estimated hundreds of thousands of troops have died on both sides, although neither release official numbers of their dead.
In recent months, Russia has been making steady ground against Ukraine's forces, although its military is sustaining massive losses.
In a surprise counterpunch, however, Ukrainian forces captured a swathe of Russian territory earlier this year when they stormed over the border and into the Kursk region.
Putin's army, with the help of North Korean reinforcements, have been attempting to retake that in the months since.
The Kursk region is seen as a major bargaining chip held by Ukraine that could be used if the incoming US administration attempted to force Zelenskyy and Putin to negotiate an end to the war.
The US is Ukraine's largest single-state backer, providing enormous amounts of military equipment and other aid. Trump has complained his country gives too much.
In a social media post late Sunday, Trump's son, Donald Trump Jnr, criticised Biden's decision, accusing the current US administration of trying to "get World War Three going before my father has a chance to create peace and save lives".
Trump is due to be inaugurated on January 20.
While he was seen as instrumental in his father's successful presidential election campaign, Trump Jnr does not yet have an official role in the next White House administration.
Analysts are sceptical about whether Biden's decision will change the course of the war.
Many Western observers believe the relaxing of conditions has come too late, and that the most valuable Russian military assets within the ATACMS 300km range will have been moved back to safer ground.
