Former Russian President Dmitry Medvedev, now deputy chairman of Russia's Security Council, made the comments in response to a visit by British Prime Minister Rishi Sunak to Kyiv to announce an increase in military funding to help Ukraine purchase new military drones.

"I hope that our eternal enemies - the arrogant British - understand that deploying an official military contingent to Ukraine would be a declaration of war against our country," Medvedev wrote on the Telegram messaging app.

Medvedev, whose frequent and harsh public statements diplomats say give an indication of hawkish thinking at the top of the Kremlin, also asked how the Western public would feel if Sunak's delegation came under fire from cluster munitions in the centre of Kyiv, something he said had recently happened to Russian civilians in the city of Belgorod.

Belgorod, in southern Russia, is located close to the Ukrainian border and has been targeted by Ukrainian rockets and drones in recent months.

In the incident referred to by Medvedev on Dec. 30, Russia said at least 20 people had been killed, including two children, and 111 injured in what it called an "indiscriminate" Ukrainian strike using cluster bombs.

Ukrainian news outlet RBC-Ukraine quoted unnamed sources as saying Ukrainian forces had directed fire at military targets in Belgorod in response to the massive Russian bombardment of Ukrainian cities the previous day.

Medvedev cast himself as a liberal moderniser when he was president from 2008-2012, but now presents himself as one of the fiercest anti-Western Kremlin hawks.