The leaders of Britain and the United States was set to meet 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 US technology but not US-made missiles themselves.

Responding to Putin's warning, Starmer told UK media travelling 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tensions, Russia's FSB security service yesterday announced that the accreditation of six British diplomats had been withdrawn and accused them of spying.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 US 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 4:30pm (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.

Washington currently authorises 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.