"All the signs are that the plan has already in some senses begun," he told BBC.

Intelligence suggests Russia intends to launch an invasion that will encircle Ukrainian capital Kyiv, Johnson said.

"People need to understand the sheer cost in human life that could entail," he said.

Meanwhile, Ukraine hopes that Russia will join the Ukraine-proposed negotiations on security guarantees within the 1994 Budapest Memorandum, but if it refuses, the talks should still take place, Ukrainian Foreign Minister Dmytro Kuleba said on Sunday.

"These security guarantees were provided to Ukraine in 1994 not from a group of countries, but by each country separately, and these guarantees were merged in one document. Therefore, Russia's refusal is not a reason not to hold consultations. But we hope that Russia will still confirm its participation," Kuleba said during an online briefing.

He also stressed that concrete decisions must be agreed upon following the consultations of guarantor countries of the agreement to ensure Ukraine's security.