"Russia will find a response to any unfriendly steps taken by the British side," Moscow's embassy in London said in a statement carried by Russian news agencies.

The Russian embassy said that London would bear responsibility for the "'bloody harvest' of the next round of escalation as well as military and political consequences ensuing from this for the European continent and the whole world."

On Wednesday, Ukrainian President Volodymyr Zelensky visited London where he urged allies to send combat aircraft to his war-torn country.

British Prime Minister Rishi Sunak said "nothing is off the table" in terms of military aid for Ukraine.

Fighter jets for Ukraine were "part of the conversation", Sunak told a joint news conference with Zelensky.