The leaders of Britain and the United States meet Friday in Washington on whether to let Kyiv fire Western-provided long-range missiles into Russia, an option that has sent tensions soaring with Moscow.
Prime Minister Keir Starmer's visit to President Joe Biden comes with Kyiv pushing for permission and Russia warning that giving Ukraine the green light would mean NATO was "at war" with Moscow.
Kremlin spokesman Dmitry Peskov said President Vladimir Putin's warning was unambiguous: "We have no doubt that this statement has reached its recipients."
British media reported that Biden, who is wary of provoking a nuclear conflict, was ready to let Ukraine deploy British and French missiles using U.S. technology but not U.S.-made missiles themselves.
Responding to Putin's warning, Starmer told U.K. media traveling with him that "Russia started this conflict. Russia illegally invaded Ukraine. Russia can end this conflict straight away."
In a sign of increasing tensions, Russia's FSB security service announced on Friday that the accreditation of six British diplomats had been withdrawn and accused them of spying.
But London dismissed the claims as "completely baseless" and indicated they were a tit-for-tat measure after it slapped new restrictions on Russian diplomats in May.
The talks come at a time when Biden is on his way out of office and November's U.S. election is a toss-up between Democrat Kamala Harris and Republican Donald Trump.
Trump repeatedly refused to take sides on the war during a debate with Harris on Tuesday, saying only: "I want the war to stop."
Starmer is set to meet Biden in the Oval Office at 4:30 pm (2030 GMT) but has no scheduled meetings at this stage with Trump or Harris, both of whom will be on the campaign trail on Friday.
His visit – his second to Washington since his Labour party stormed to victory in July after 14 years – is also aimed at papering over differences on the war in Gaza.
Starmer's government last week announced restrictions on some weapons to Israel, voicing concern that they could be used to violate international humanitarian law.
The White House has declined to criticize Britain's decision, but Politico reported that Washington had asked London what it would take to change its decision – with the answer being a cease-fire in Gaza.
But Ukraine will be the main focus, with mounting concerns over the country's losses on the battlefield more than two-and-a-half years into the war.
Biden said on Tuesday that he was "working" on Ukraine's demands, while top U.S. and British diplomats Antony Blinken and David Lammy made a rare joint visit to Kyiv on Wednesday.
Blinken promised that Washington would now quickly review Kyiv's long-standing request and would "adjust, we'll adapt as necessary" to help Ukraine defend itself.
Washington currently authorizes Ukraine to only hit Russian targets in the occupied parts of Ukraine and some in Russian border regions directly related to Moscow's combat operations.
But Putin, who has rattled the saber of nuclear conflict since the start of his February 2022 invasion of Ukraine, warned the United States and United Kingdom against such a move.
"This would in a significant way change the very nature of the conflict. It would mean that NATO countries, the US, European countries, are at war with Russia," he said on Thursday.
Biden has strongly supported Ukraine since Russia's invasion to the tune of billions of dollars in aid as well as political capital at home.
But he has been risk averse about stepping up to new kinds of weaponry deliveries – with Ukraine having to wait until this year to get F-16 jets.
The looming U.S. election means the clock is ticking, with Kyiv in particular eyeing a Trump presidency with trepidation.
Trump has long been lukewarm on supporting Kyiv, and has frequently praised Putin.
In his debate with Harris on Tuesday, he pledged to get an agreement to end the war "before I even become president" – a deal many Ukrainians fear would force them to accept Russia's territorial gains.
Vice President Harris has in contrast pledged to keep up staunch support for Ukraine if elected.